Based on my own commuter/utility/fitness hack bike, the specifications for a useful machine are:
Clearance for 32mm tyres + fenders using long drop caliper brakes.
Eyelets for rack and fenders.
Handbuilt 36 spoke wheels, but these need not be made of all premium parts. My Altus hubs work just fine.
A good headset and bottom bracket.
Gearing to suits your conditions. Stock racing gears are too high. Road triples are usually OK. You can be creative and use a low-ration double (36/48) or a mid-range single chainring.

There are some nice stock frames that are ready to build up. Workshops like Gunnar, Soma and Surley do good value light-touring or cyclo-cross/touring frames.
With used bikes, there are so many, but look for some quality in the frame construction. There is no excuse for riding a crappy used bike when there are so many good cheap ones..