For about $120 more (including shipping), you could get a Wabi Classic. Eighthinch is still doing complete Scramblers for $499 shipped ($449 if they're still taking the "rabbit" coupon code). That would leave you with a few bucks for some immediate upgrades. You'll probably get a lot of suggestions for Kilo TT on here, which would leave even more cash for upgrades.

Have you tried perusing Craigslist? Depending on where you live it'll provide some affordable, quality options. Also, what's the terrain/road like on your way to work? You looking at terrible cracks or smooth riding? You might consider getting a "beefier" frame that can take wider tires if you expect lots of bumps (e.g. Kilo WT, Motobecane Jury). Otherwise you could tarck one out with the slimmer tires. Just something to consider.