That ebay bike would only require a BMX freewheel to be a coaster. If you go with a size cog that is enough different, you may need a new chain too, but your bike shop could do that for you in a couple of minutes.

As far as worrying about your size, just stick with wheels with 32 or more spokes, tires should be 700x25 or larger (700x28, 700x32 if room). I started riding fixed at about 315 (down from 365), now I am 280. The only difference I have had to deal with is slightly wider tires, and I do a lot more dammage to a car when they decide to collide with me.

Oh yeah, don't skimp on pedals. Get some nice, strong, NEW ones. If you searched for it, you could probably find the results of what happened when I kept the "classic" pedals on my conversion.