I'm in the same position. I've got a good friend (ex-wrench) that's telling be to piece one together from tons of used bits but my head says to just buy a Surly and be done with it. Interestingly, he did say that if I go surfing eBay NOT to buy a track frame and convert it because of the geometry.
If you aren't afraid to be seen on a nice old lugged recycled steel conversion, I would strongly suggest that as the way to go. You will end up with better components than if you buy a me-too off the rack $500 neo-"track" bike. You will also learn a lot about wrenching. I went so far as to build my own rear wheel. I ended up with an awesome conversion. Altogether, it may well cost as much as the cheapest new fixed gear, but it has much better parts.
Total control over all parts decisions
Unique finished project
May need to buy lots of tools that will become obsolete after the build
Potential difficulty in finding some parts, depending on frame age/style
All the parts fit and work well together
Good relationship with LBS
You're getting a known quantity/less guesswork
Not labor intensive
You get the bike faster (usually)
Substituting parts may be difficult and/or expensive
The manufaturer makes the parts decisions for you
Bike is like lots of others
Can be more expensive
What I did was compromise. I bought a Surly from my LBS. QBP (Surly's parent company and distributor) lets you choose from a vast array of parts with which to build it, as long as you order a "complete bike" (that is, you get every single part needed for a complete bike). It arrives at the shop in several boxes: frame & fork, wheels, and parts. Then you have a choice; either let the shop build it or build it yourself. Or compromise again and have the shop press in the headset cups and thread the BB or whatever aspects of the build you don't want to or can't do yourself, and then you do the rest.
EDIT PS: I wouldn't reject a "track frame" outright, but be aware that the angles are pretty steep and the fork probably won't be drilled for a brake.
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