If you are living in Center City or one of the surrounding areas (W. Philly, No Libs, South Philly, etc) a single speed is fine as the city is pretty flat. There are a few minors hills, but nothing standing up and pumping can't overcome. I just sold a single speed commuter I built out of an old panasonic. It had a 52t crank and I think I was using a 16t cog in back. Worked just fine for me and I rarely topped out between lights. Surlys are not uncommon sights in the city, but I'd consider where you are going to be leaving it and for how long as bike theft is high right now. I had my empty crappy 10 buck saddle bag stolen off another bike when I left it outside overnight in Grad hospital area. My advice is buy the meanest lock possible (I wear a Kryptonite New York Fahgettaboudit slung over my shoulder when riding which has the double effect of calming down would be honkers) and invest in locking skewers.

Las thing I'll mention is regarding tires. We got a ton of snow this past year so if you plan on winter commuting plan to have a bike that will take salt, ice, snow, and hydro-planing (its so much fun sliding into parked cars...). I run 28c or higher on 700 wheels.

PM me if you need more info or want to know about specific neighborhoods, routes, or anything Philly related actually.