I had a few Chevy's in my life and expereince with maintenance(some work I did myself-some I took to shop) on these kinds of cars:
1: (critical)Have them move the car and check for oil/transmission spots. Oil brown spots where the engine is, transmission fluid red/burnt brown where transmission is. CGM's mid 70's to late 80's were sort of bad with rear engine seals leak. What that means is that the seal in the back of the engine is decaying and the engine is leaking oil. You will alway shave to add oil, and it's just a matter of time before they completely decay, yoru addign oil all the time, and your engine is losing oil pressure. Also means that when your car is at a stop and it's dripping oil on a hot engine your car is going to smoke.
2: (critical) Start car, let run for 10-15 minues on a level surface and pull out the transmission stick. Check the color of the fluid on the stick, should be bright red, if it's burnt or brown you may have SERIOUS tranmission problems in the future. Transmissions can be rebuilt for 4 to 6 hundred dollars, but if the torque convertor is gone, add another 4 to 6 hundred.
3.(critical) while car is running put foot on break and place car in reverse. Listen to the way the transmission shifts, is it smooth or do you ear a "klunk" sound and a sinking feeling when you put it in reverse. If it klunks tell the owner you wanna take it to a shop and have the transmission pan pulled off. the tell tale sign will be metal shavings in the bottom of the pan, broken gasket seal, dis-colored fluid. Ask the mechanic that takes of the pan about all of those things, also about the torque converter.
4.(serious) Right above teh spark parks on both sides of the engine is where the valve covers are. Look for dirt deposits and old oil around those areas. If you see dirt run yoru finger through it and see if oil is present. If so the valve cover gasket is broken and teh oil running accros the valve is leaking. Not to expensive to get fix just a coupel of hundred if you pay someone, I use to do it myself. Sort of easy to do if you have tools.
Anymore questions send me a pm homey. I had a 76 Impala, a 84 Monte Carlo, and a 94 Monte Carlo in my life time.