We've been doing that to our 1977-78 house up here in Alaska for 15 years, hope you're ready for a long haul of open walls/ceilings and sound transmission everywhere
For the "resale value" (we never plan on selling, but that's apparently a thing for everyone else in the world) we did a major remodel of the kitchen; moved all the appliance and plumbing, all new cabs, etc. (Have an album up with products if your interested.)
For the "living comfort/saving money" the best thing we did was replacing all the old windows and sliding doors with low-e. Replacing the windows and sliding doors was the very first thing we did and it made a HUGE difference for keeping the temps inside stable and comfortable, and I think our NG bill cut in half from it.
-- Frankly, if one was only doing one thing, new windows is 100% my pick. Our windows used to frost up in the winter (it gets -20*F or lower pretty regularly up here) For Xmas I got an inside/outside thermometer that I put on the kitchen window sill and I've been watching it all winter. I've never seen the temp go below 60*F inside (we keep the house at 65*F) I'm super impressed with the low-e's, and they're not even top quality, just Home Depot vinyls (we skimped a bit there cause I wasn't thinking about the remodel thing with the same frame of mind at the time.)
Second best for "living comfort/saving money" was that during the kitchen remodel we had pulled the exterior siding and before putting the siding back on we put on exterior house wrap (Tyvek.) Between the wrap and windows there was a MASSIVE difference in how hot it would get downstairs here (we get about 9-10 hours of full sun on a dark chocolate brown exterior all summer long, it could get really miserable.) Now it stays far cooler during the summer, and even though we pulled out nearly half the hot water baseboards downstairs, it still stays warm too.
Wall-o-text of things I'm super happy we've done/are doing that you might want to consider:
New boiler and hot water heater, I can't even say how pleased I am with the "near instant" hot water and huge hot water tank we put in. I take hour long hot as it'll come out showers and the setup we have handles it with ease. We got a Munchkin boiler (husband says they went under? IDK why their boiler is absolutely amazing. 16 years installed and never had a single problem with her, AND she's 100% zero maintenance.) I also love the Ultrastor brand indirect fired water heater we got; no need to worry about replacing the rods or anything, it's nearly maintenance free and just perfect IMO.
Lights in the "living room" - the front room had a switched outlet but no ceiling lights. We'd put our pool table in there and it's been basically unusable all winter long (we only get like 5 hours of daylight in the winter.) We're currently putting in overhead ceiling lights and pool table lights, it's gonna be lovely.
Running internet & cable to every room - it's gonna be great since the cable company doesn't put them in-wall for you. If you've got the floor or ceiling open it's nice to run those. I'm running HDMI's & 3.5mm IR cables (they're headphone cables, but you put IR on them to relay remote control signals to your receiver/TV/other component) from the upstairs office/home theater to the downstairs game room and kitchen so I'll be able to basically do whole house audio and video. I'm installing 13.2.4 ATMOS speakers in the office/home theater. I also put an extra coax and Ethernet cable into the basement for later expansion to the dining room or back porch. I'm super excited to be almost done with all that.
MOAR OUTLETS! - IDK about your place, but mine never had enough, nor were they well located, so while we've got various floors/walls/ceilings open I'm adding them in or relocating them so they're more functional. Also running an outlet + 8K HDMI + TosLink cable up to all our wall mounted TV's to get rid of hanging cables and/or cable tracks (which never look right to me, even if you paint em to match the walls.)
Related to above - we're replacing all the old outlets in the house. We've been doing the decora style Leviton tamper resistant ones [for grandkids which are coming soon,] but I kinda want to put in some of the new square Legrand Adorne style ones cause they're so cool (they make some nice looking switches too, JS) - Also might want to swap out some outlets for ones that have the built in 5v USB ports on them for phone charging.
Also related to the above - Garage door openers weren't really a thing until the 80s. I don't know about you, but the orange extension cord across the ceiling thing just doesn't work for me
The garage door opener oft uses 22/2 (same as the door bell) I've been running 18/4 just for GP. (Might consider installing some laser parking assists while you're at it. We really love ours. There's a number of good ones that work in conjunction to the safety sensors and don't require an outlet, or bulb to outlet adapter as new openers tend to have LED's these days.)
Also, also related to above - we're putting in some outlets and switches that have built in night lights in areas that are "high traffic" so we don't have to turn on the overhead light. So far we've got night light switches at the top and bottom of the stairs and in the dining room near the garage door. Also put in nightlight outlets in all the bathrooms as well as the hallways (which originally had exactly ZERO outlets.)
Changing out switch wires - We've got 12/2 through-out the house, but I'm running 12/3 to all the switches we get access to. We probably won't need it ourselves, but the newer "smart" switches use the third wire as I hear it (also might be code now?) so we're putting them in anyway.
Changing out bathroom fans - the new ones are far quieter, typically move more air, the lighting is oft brighter/cheaper LED, AND the new ones are so much more attractive. If we're able, we're also swapping out the 12/2 to the switches to 12/4 for those ones that have fan, light, and night light on separate switches.
Changing out ceiling lights - All the lights in this place were pretty poor at lighting so we've been switching them all out as well. We've been putting LED panels in the hallways and other high use rooms- we're very happy with that change.
New moldings - We had/have that typical 3" contractor base molding through-out the house and it's so boring. I'm switching it all out for taller 4 to 6" base molding cause it just looks so much better. Also crown is really nice depending on your interior styling - I'm going to be putting in crown with LED strip in/on it as accent lighting as well. (Hard wiring that up can get a bit more complicated if you get the typical 12v stuff as it needs a transformer and stuff.)
Install hard-wired smoke & CO2 detectors - I like to have a hard wired one of each flavor on each floor, then I supplement with the battery ones to meet code. (They make ones that do both now, though I've not researched a hard-wired combo one.) Also think about running any security stuff cables/wires while you've got walls/floors/ceilings open - there's motion detectors, break sensors, and cameras; I've been running RG9 Coax cable for video, and 22/4 to feed any of those things I think I might want to have down the line.
Eave lighting - I'm pretty sure that eave lighting is the best thing in exterior lighting ever. The setup I'm going to be doing will be individually addressable RGB strips so we won't even have to put up xmas lights anymore, just code the Arduino to do basically whatever I want, from color to chase and fade and everything in-between. It's gonna be glorious. Hard-wiring for my setup requires a 22/4 through the exterior wall to where I'm putting the controller and power supply inside (the upstairs office in my case.)
Change out exterior lights - We replaced all our old flood lights with motion sensing LEDs and I can't even find words for how much better it is. -- Related; eventually we'll be running flag/yard/porch/driveway lights too.
Change out the doorbell - I'm just going to replace my entire doorbell setup, wiring and all, cause it not only sounds awful but it only works half the time. If you're thinking about a RING door bell you can run the standard 20/2 doorbell wire for that (and make sure you get the RING II that can be hardwired.)
I think that's quite enough from me so I'll shut up now. I love this stuff :vs_laugh: