Avoid French bikes. Depending on how old it is, you might have a hell of a time finding replacement parts if you need them.
If you're going used, ideally you'd want to find something where the conversion has already been done. You _could_ purchase an old 10-speed and have the conversion done at your LBS, but parts and labor would bring the price near your limit.
Of course, BikesDirect offers a number of single-speed and fixed gear bicycles for about the same as you would spend on an Angus. Personally, if I had no interest in actually working on my bike, I would purchase something (new or used) from a LBS. You'll build rapport and get the amenities like free tune-ups and whatnot. More often than not, you'll get some attitude from the people at the shop if you bring in a brand new bike that you didn't buy from them. And you'll get gouged on the labor.
Last edited by striknein; 03-09-11 at 11:55 PM.