I've been looking for the stuff I bought a year or two ago, Bullson 2 piece (yes I know you said one piece), it was as cheap as you like and has never let me down for either road or trail riding.
I've used one-piece suits in the past, and they are a PITA.... not easy to put on, especially over damp riding gear (lets face it, we on;y put this stuff on after it starts to rain in most cases).
If you have reasonably waterproof (showerproof) riding gear, or even treated leather then a little water ingress isn't going to matter too much. Two piece suits are easier to put on, take off and if you burn a hole in your pants, you don't need a whole new suit!
Unfortunately, it appears that Bullson rain suits aren't available any more....
