Detached garage heating/cooling

I am purchasing a new home that includes a detached garage. The builder calls it a 400 square feet garage, although my measurements were 24'x15', which in my calculation is 360 square feet (25’x16’ would be 400 square feet, maybe they were using outside dimensions while I was measuring internal dimensions). I talked (and paid for) the builder into insulating the garage unit; I will also insulate the garage door. I also had them install a dedicated 240V/30Amp circuit on the backside of the building, to allow for an air conditioning unit; their options for 240V only included 50A or 30A, so this should be overkill. I'm looking for a ductless mini-split AC/heater.

My plan for this room is for it to be my computer/electronics lab/“man cave”. I will be installing a 42U computer rack system, which will have several servers running 24/7. If it makes any difference, I plan on putting down non-padded carpet on the concrete floor. I do not know much heat the servers generate, but there will be at least three enterprise-grade servers plus about 5 desktop computers in there. I live near Sacramento, California, where from June through September the average high temperature is at least 90 with it frequently hitting 100 in July and August. I'm not nearly concerned about the heating side, as computers generally don't have issues with cold; granted, if I'm spending time in there, I will want to be comfortable. In December and January, the average low is around 40, although it does get below freezing several nights if it is clear and dry.

As I do more and more research, I am finding more questions than answers. I find that these ductless mini-splits are usually labeled as “Inverter” units, which to my understanding allows it to ramp up to the needed cooling instead of being all on or all off. Then I heard the term “oversizing,” just to try to confuse me further. One article I read on oversizing talked about not going so overboard on oversizing the system for the needs that the minimal capabilities of the unit is higher than the cooling needs. The author of the article mentioned that the operating range of a 9,000 BTUH inverter evaporator is ~2,000-11,000 BTUH, while a 12,000 BTUH has a range of 3,000-13,000 BTUH, and lastly listed an 18,000 BTUH unit’s range is 7,000-23,000 BTUH. I believe I read that you would like to have your normal needs around the low end of this range for optimal efficiency but not below the low end. With my room being approximately 400 square feet, it looks like the recommended amount of cooling is around 10,000-13,000 BTUH, depending on different factors I’ve seen on calculators. To me, this makes it look like I ought to go with an 18,000 BTUH unit.

As I’m not completely stupid, I would like to pay the least for the best system I can afford. I've called a few HVAC companies and they are quoting me systems around $6,500-$8,000. They were talking about units from South Korean or Japanese companies (LG or Fujitsu). I have also looked at units presumably made in China but having American-sounding names put on them, despite them all looking alike. I was first looking at a company called Alpine Home Air who is selling a unit called BlueRidge. Another I’ve seen is Pioneer. The last one I’ve been seeing everywhere is MrCool. The interesting thing from McCool is I see some marketed as DIY and other marketed as “Advantage.” The DIY model is significantly more expensive than the Advantage model, but it appears the SEER rating is higher on the DIY and the warranty is longer. As I already have a 30-amp 240-volt circuit, which is overkill for any system, I don’t need to scale back to something designed for 120-volts. Also, I’m not scared of doing the physical installation work, I’ve done electrical work before, but I have no experience with charging/testing coolant lines. It appears that some of these “off-brand” units recommend the final coolant line stuff be left to a professional and others are designed so a professional is not needed. The MrCool DIY series looks as if a professional is NOT needed, but it looks more expensive than the ones where a professional is recommended. One of the local HVAC professionals I talked to said that they could come out and do the checking of the system but that charge would be about $1,200!!! That is more than the unit costs in the first place and I would have completed MOST of the labor before they arrived!

My big question is whether paying $6,500-$8,000 for a quality brand installed by a professional is worth it over a MrCool DIY unit costing ~$1,300. That is a HUGE price discrepancy! MrCool has a 7-year compressor and 5-year parts warranty… that seems good enough for me. I’m just hoping to get feedback from someone away from or towards a product and if you would agree an 18,000 BTUH unit is appropriate.