Buying a quality bike within your price range can be exceedingly difficult. Buying used will require some prior bike knowledge, unless you buy from a reputable bike shop.

You should first consider your terrain, after deciding your budget. If you have mostly a flat landscape, then a single speed would be more suitable for you. That's only because they are usually less expensive and require less maintenance. For the Dawes SST single speed, go to the following:
www.bikesdirect.com.
(Talk to their customer service dept. about sizing, before ordering)

OTOH, if you do have hills or long inclines to contend with, a used CROMO steel road or mountain bike from the 80's era should do just fine.

*In any case, if you have doubts about the fit, don't buy it!