You may be "comfortable" on a 56 but it may not be optimumal for you. A lot depends on your inseam measurement. In any case, a vintage mtb like this would make a great commuter at a much lower price:
http://seattle.craigslist.org/sno/bik/4042166940.html
And while I rarely disagree with oddjob a/k/a the oracle as to price, I think $500 is too high for a 710 that needs a fair amount of work. That's an OK price but you can do a lot better by shopping hard.