I would say:

- avoid older Schwinns and French bikes with non-standard threaded components
- avoid everything with more than a bit of surface rust
- unless you up for truing wheels, avoid everything with problematic wheels
- avoid everything with one piece cranks
- when buying a bike, bring along some allen wrenches. Try to remove the stem and seatpost. If either is stuck, pass on the bike
- avoid department store brands (Huffy, Murray, Free Spirit -with few exceptions)

It is true that hi-tensile steel, turkey wings, chain protectors, stem shifters etc do denote an entry level bike; but entry level bikes would be fine for your purpose and as a matter of fact, since the acquisition $ is low, they might be what you should be look for your first project instead of (lets say) a Colnago.