mostr, your house sounds a lot like mine, which was also built in the 1970's. Is your house a split-level? The half of the basement that was 'finished' when we moved in has 1"x2" boards between the cement and the drywall. No insulation and only 2 outlets for the entire space. It'll be gutted one of these days. It would be interesting to see pictures as you start cutting through the cement for the bathroom.
We have finished the other half of the basement. Here are a few things I didn't do when that space was open that I wish I had done, maybe you'll want to think about them (or already have - sorry)--
-spray foam around all the rim joists before insulating.
-metal tape all the seams in the duct work and insulate the ducts.
-soundproof the recroom (or you'll hear, "turn the volume down!")
-phone and satelite/cable outlets on every wall (so you can re-arrange furniture/tv)
-extra electric outlets
-get the necessary permits
-1" extruded foam board under 5/8" t&g plywood on the floor for warmth.
BTW, I'm in no way an expert on any of this stuff, but those are a few things I didn't know about before I started. Except for the permits, I skipped that on purpose
. I got one to build a deck outside, but not for a bedroom and family room I was finishing for my grandson to live in when he was born - very stupid!