I'd spend more money up front and get a new bike in the $500+ price range if you can swing it. Otherwise, finding a used conversion on Craigslist is probably the best bang for the buck.

Buying parts individually and/or upgrading can get expensive really quickly. You'll get more value if you buy a good complete bike or a used one fully built up, instead of buying something cheap and trying to upgrade it part by part.

Some of the best values on Craigslist are older road bikes that would make great conversions. The fixed gear craze is kind of big right now, so FG bikes seem to command a premium that older road bikes don't. Of course older road bikes won't have a sloping frame geometry, so that may not appeal to you.