I own a 74 LS4/Automatic Coupe. I bought it about a year ago for $17,000. It's NCRS Top Flight, AC, Pwr Windows, Leather, AM/FM Stereo, etc. Overall, it is a very nice, quiet, tight, car. While only 270 HP (Net), the car runs very strong and has a ton of torque!
Here is what I think you need to look for...
1) Is the interior hot when you drive it? This is a big deal if you are not capable of taking the dash apart (far apart) and rebuilding the heater box, RS Cowl Plenum Vent, Heater Core, gaskets, and bunches of other small parts. This can be a near fatal case of the "While I'm in there, I'll fix this."
2) Does the AC work?
3) Brakes, $1,000 -$1,500 to fix them yourself
4) Frame, Suspension, Front Cross-member, Rear Trailing Arms
5) Exhaust System
6) Interior... seats, carpeting, dash pad, door panels?
I was fortunate. My car had a frame off restoration in the early 90's and all I have had to do is #1 & 2 and can do the work myself.
If you are not real handy, you may want to consider an early C4 which can be purchased in pretty nice condition for $8,500 - $12,000.
I like my car... but it is a LABOR of love.