It depends on a variety of things, such as how much money you want to spend or how nice the frame is. For example, my first fixed gear was a buildup of a decent, but not great, old japanese frame. I basically put on the cheapest stuff I could find and it has worked out fine--sun rims and suzue fixed/free hub. If you can, I would recommend getting the wheels done at a local shop so they can help you out with chain line, etc. For my next build up I am going to put on better stuff and bling it out a little.
That having been said, get a Level hub for the rear. Those are awesome. The cogs bolt onto the hub like a chainwheel. Very cool. For vendors, if you search the forums you will find loads of different opinions on that.