Fuji Track has a very nice double butted steel frame and has good geometry for the road. Beware of “cool” boutique brands who do not make their own frames – especially generic imported and unbutted tubing, all you are getting is image and flash stickers on a crap frame.
Pompinos are good frames, but not the lightest or prettiest. Their big advantage is they use v-brakes, you can fit racks, and there’s room for fatter tyres, so they are useable offroad too. Probably the best fixie for a person with mountain biker habits!
In alloy, the Giant Bowery has a nice feel and a very direct power delivery. I would buy one of those if I didn’t have too many bikes already. Sometimes these are available at bargain prices.
I would never convert another old cheap steel geared bike. Their chainstays are often too flexy for steep climbs – you can flex it so much that the rim will rub on the brake, or if no brake, on the chain stay. No good if you are trying to climb. Ok if you don’t have steep climbs maybe, but generally a PITA otherwise, and not noticeably cheaper than getting a proper frame secondhand. It can be a very satisfying project though – look how nice Alpin’s bike is.
Warning: I’m biased – I own a Fuji and Pompino – but that’s really a case of putting my money where my mouth is.