I've been amazed at how easily old bikes clean up. Buy yourself a tub of grease, repack all the bearings (if they need it) and you're good to go.

If you look at older steel bikes, here's what I look for:

1. Frame- make sure all the tubes are straight (or at least symmetrical if they are meant to be bent), make sure the fork is straight and that there is no peeling paint or bulges just behind the head tube that may indicate a severe crash. Look at the crank area, the top of the seat post, and the dropouts where the rear wheel is mounted to make sure they are all intact.

2. Wheels- a little out of true is easy to fix with a spoke wrench. Stay away from obviously bent rims, rims that are badly out of true, or rims that are out of round (i.e., they have a "hop" or a flat spot). Make sure you look at the rim itself when you check this because tires can be all over the place even on a straight rim. If the wheels don't spin easily, that's not necessarily a big deal; just repack the hub bearings (but wheels that don't spin well are a good bargaining chip in your favor).

3. Gears- make sure they shift. If the cables are seized up, move the mechanism by hand at the derailleur. If the chain is slack, it may be just that the rear derailleur needs some clean and lube, or maybe the freewheel is gummed up. If it's the freewheel, you can try just oiling it from the front without removing it from the hub.

In the 1970s and 80s, Suntour was king. Much more common than Shimano and in my opinion at least as good as Shimano if not better. I have several Suntour-equipped bikes and they work flawlessly, even after 20 or 30 years.

I've pulled some real frogs out of dumpsters and trash heaps and with a little polishing and lubing they become handsome princes. It takes a few tools, not many really. My LBS is just as happy to sell me tools (and tell me how to use them) as they are to sell me bike parts and accessories. For tools I will rarely use, the mechanic will just loosen up the nut or freewheel in question without charging me (if it only takes a few seconds, which a lot of these fasteners do if you have the right tools).

About the worst thing you can do when working on an older bike is strip out threads. So be careful with that, and the rest just kind of works itself out. Read up here or at Sheldon Brown or Bike Tutor or the Park Tool site.