Buying used without at least a mediocre knowledge of what is out there, the history of some companies, components etc. is a crap shoot. If you know someone that can explain what your getting into with a used bike, its a good deal.

Case in point: I've heard of people here paying well over $200 for a used Schwinn, but then I've seen a Bridgestone RB-1 and a Lemond of unidentified origin both sell at pawn shops for less than $50 each. Do you know what the various brands of components are, and what (say 1993) offered in top end, mid-range etc? Do you know the difference between the mountain bike and road bike offerings?

I think you sound like you want to do the commute, and that's the first step. The problem you seem to have is that you're limited in having someone explain to you what you need to make this happen.

My honest recommendation. Find a LBS, go in and chat with them. Explain your intentions and see what they recommend. They probably will be the safest guides.