Boats from the 70s don't have the blister problems that 80s boats have. That said, with a newer boat all systems are new.
I live on a 30, similar to a catalina 30 or so I've heard. The surveyor gave it a fair market value around your range, but the "market" now is not "fair"! You may find yourself getting the boat you're looking for less. That catalina you're looking at better be in great condition with a few expensive extras like a roller furler
& dodger. Craigslist and ebay are good places to look for yuppies that are too lazy to get a good price.
I wouldn't live in smaller than a 27, but to each their own. Head+holding tank is important. I fill up a 12gal tank in no time.
I know for a fact that marina living can be far, far, cheaper than dirt dwelling in my area. You can get a 33' slip for $3200/y, power+liveaboard $115/mo, in a neighborhood where you can get a crappy appartment for $1k/mo!!!!!! There's a lot of incidentals but most of them add value to the boat, and the entry fee is steep. What's more, if you like the life and can't relo, you could might take to a condo marina, lay down 20 grand and get a slip forever, rent income when you're out.
What you're looking for in a liveaboard is
-hull liner, overhead liner, insulation. enough of it and condensation is not a problem
-AC outlets with big amperage, if you're gonna run space heaters. mine didn't have enough wiring to keep warm!
- big holding tank
, water tank
- less teak exposed to weather
- nothing crazy like a wood hull
- cooking facilities, propane is best
-engine, diesel is best
- icebox. if it's well-insulated, check to make sure you havent' slipped into a bizarro alternate universe. marine refrigerators are $$$$$$$$, big big bonus
You don't seem to be looking for one, but i think it's worth saying, it's a bear to live on a "project boat". I wouldn't take the introduction to living aboard and advanced boat repair
on the same day.