For the money, it may be better to do a conversion. Then again, I understand new bike lust.

I find it doubtful that you could build up a Steamroller for less than $500. $350 for the frame, $90 for a cheapo rearwheel, leaves $70 for seatpost, saddle, headset, front wheel, stem, bars, brake, bottom bracket, cranks, chain, chainring, cog, and lockring (or freewheel). If you had any of those parts on hand, you could do it, but that's pretty much always true, right?

As for the brakes, you probably still only need one. The front brake can effectively deliver maximum braking power. Reasons for having a rear as well would be things like being afraid of pitchover, excessive rim heating (long braking descents can actually cause the tire to burst due to rim heating), or other control issues related to having the rear roll more easily than the front. Some run just a front on single speeds though myself I like to have a rear brake for the control aspect.