You are going to have a lot of work ahead of you, but it's mostly conceptual and observational.
Here's some very broad bits of advice:
1) 1970s and even some well-maintained 1960s FG boats are still good for sailing for many years.
2) Most racers from this period have been worn out by now, and greatly superseded by new designs and materials. There are some exceptions, like with Sharks, CSes and C&Cs, all of which have enough critical mass of boats to get a competitive PHRF rating. But you don't say whether you wish to race.
3) A very important aspect is care and maintenance. A single owner with some skill might have bought Typical 30 foot Cruiser in 1975 at 40 and has had it put on the hard...properly...every winter since (In the North East, you have to think that every boat is only X years old times 7/12ths. Against this lighter use are the structural strains imposed by craning and cradling and the potential for leaky cores freezing and thawing every year, destroying the bonds). Anyway, said skipper is 72 now and is maybe looking to pass the boat on to a responsible buyer. That's the sort of deal you should look for. Well-cared for boats have frequently been repowered once and electrically upgraded twice or even three times in 32 years.
4) Look to the kind of sailing you'll do. 1970s boats were influenced strongly by the CCA and IOR design rules. Not all of them, but a lot of the production boats are masthead sloops with large J measurements, skinny, tall mains and big, overlapping genoas. While this is a great set-up for beating to weather with a crew, it is trickier to sail solo, where a more modest rig with a 100% genoa on a somewhat more modest J measurement is going to be easier.
5) When a fibreglass boat IS too old (due to cost-cutting measures in the later '70s and early '80s, some of those boats are "dying" sooner than those 10 years or more older), the signs are clear. Get Don Casey's book on evaluating the older sailboat (can't remember the exact name) and rely on a surveyor to guide you if you see what you like. Also, crew on an older boat in a race situation. It will show you the limits of the design, and even if you are a dawdling day sailer, it's good to know those limits.
6) Lastly, a lot of older boats are pretty primitive and "brown" down below. If you figure everything's good but the wiring (which is sometimes household grade and half-rotten and insufficient for current uses), negotiate that down. Better a "bare" Good Old Boat (a good if pricey resource magazine, by the way), than one where mistakes have just been soldered to other mistakes.
7) Don't fear the Atomic 4. A well-maintained one is good for 50 years.