I used to ride a fixed gear for a few years, now I mostly ride my Cafe Motorcycle. But from my experience I prefer riding conversions over track bikes. Mostly because road bikes have that bigger wheel base and no toe overlap, so its a much more comfortable ride and much more stable. Riding track bikes off the track sorta sucked for me because they were really twitchy, unstable, uncomfortable for long rides, and you had to pay attention to the toe overlap.
And on chainlines ultimately your never gonna get as straight of a chainline from a conversion as you would a track bike, but keeping your chain on your conversion is pretty easy too. Like I've said I used to ride my conversion for years and I only jumped the chain maybe twice, both times because i had just re-lubed the drive-train and i didn't tightened shit down enough. If you just learn how to skid right and make sure everything is gonig to be as straight and tight as you can a coversion works just fine, and in my opinion better than a track bike, atleast for riding in the city.
And to the author of the thread: if you're looking for the best deal buying a pre-made bike is the easiest and possibly cheapest for a decent bike. But i strongly suggest that you part one together and learn how to build bikes so you can fix your own when the time comes, plus its a lot of fun to build them.