Most of my old tools are old ones I pick up and rebuild. There is not much really to rebuilding any. Just time,oil,grease,paint and grit. Once you do get it look for the model number, a lot of the craftsman stuff will be like 109.XXXXXXX then look on owwm for the manual. Its a good site but don't get too involved. I quit going there due to some of the mods overthere, I got tired of their BS.
Anyway as far as what to look for straightness or warping, cracks and parts missing. Parts can be had at ebay but pricey. I tend to stay away if it has parts missing that would be hard to find unless its for parts only. Belts,wiring cords and some switches can be got anywhere. Pulleys are not too bad to find. You can get new pulleys at places like grangers I think its called. I rebuilt a delta lathe and got a pulley there. But cast iron parts are harder to find. The clamps that hold the tray sometimes crack so look for welds. If you rebuild it you can clean any aluminum parts with oven cleaner or dip them in water that has lye crystals you use to clean drains. But watch closely so it will not eat through the aluminum. And also wash thourghly with cold water when done. If the aluminum has decals or factory paint you want to stay don't do this method it will remove it. I poliish it with scotchbrite then a silver wadding that you can get at walmart in a can.
Krylon has a good primer and also a good match to the gray paint used on the older tools. They also have a good blue like color for the deltas and some craftsman as well. I have redone tools for years and the krylon rattle cans does great if you do good prepwork. If you have rusted together parts I electroylitic to clean them. you can do a search on that.Don't use that for anything plated or has wood on it. Plating will come off and wood will turn blank. I have done tools that are comepletely rusted up and got them to going again. They are not bad to do either, My scroll saw below I did in about a week in spare time. I only have 80 bucks in it and I couldn't get another deep scroll saw for as cheap or good quality anywhere else.
To give you an idea of what can be done here is a before and after shot. I had to make a few of the parts used to tighten the head as well. To see some of my tools check out the pics at
http://good-times.webshots.com/album/569408649spoWff
Now the saw,before and after
Before
And the after
Pictures just not do an old tool justice after it has been redone. Let me know if you have questions, I have rebuilt and fixed my own tools for over 30 years now. Its the best way I know how to get good quality tools on a budget. Now if I can only find a small metal lathe.