I had a 240 guy with me once (bow up attitude): on a mirror smooth lake (Lillinoma) I got 52mph! I think it was "perfect weight and water condition."
I shopped around at marinas for one first. They wanted around $500. Ended up buying one from one of the big internet catalogs. I think it cost $160. I put it on myself. Took for screws and 30 minutes.