Veloswap is good, but Craigslist is cheaper IME. It's funny because I ride a 58cm or 59cm frame and all I can find when I look are 54cm and 56cm for short people. :-)
FWIW singlespeeds are not track bikes, which are fixed gear. Track bikes don't free wheel. The reason people run fixies and singlespeeds is simplicity. You'll find commuting is tough on gear and the less stuff to maintain or break the better. It's a trade-off. I agree, fixies are done to prove how cool you are, but bike messengers run them because they really do break a lot of stuff and freehubs are notoriously prone to explosion. But mostly it's hip.
I run a cyclocross bike as my commuter. From Sept to Dec it's stripped since I race it on weekeneds, but the rest of the year it has fenders, lights, bottles and all of it. I use a backpack, I am not a fan of racks and panniers. Some people (my wife, Ricardo) love them. Panniers are a pain on buses, leave them on and your stuff gets wet, taking them off is time consuming and if someone takes your bike all your stuff goes with it.
Oh, regarding level of quality. You're commuting into Boulder, I would use the lowest level that gets the job done because bling and expensive stuff attracts theft and the bike thieves up there do know what is good and what is junk.
I'd find a few year old MTB, so look for a medium, 17" or maybe 18" for you. Run skinny slicks, 26x1.5 or 26x1.9 and call it good. You can run drop bars if you want them on a MTB and they are more suited to the daily grind.
'91 Pickup - Imelda / '08 Tacoma TRD - Donna
'09 Kawasaki KLR650
'98 Rhygin Juke SS / '12 Gunnar Rockhound 29 / '16 REEB Dikyelous
"Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security. " --Albert Einstein