Welcome to Ribnet!
Like Spartacus, I have never heard of that make, althoguh a quick google search they seem to be more famous for liferafts?
Anyhow, 20Hp on a 5m might be OK if the hull is exceptionally light and isn't too deep a V (at risk of starting the inevitable "discussion"...
). I had a 25 on the back of my SR4, and in the dim & distant past spent a couple of days with a half dead set of twin 30s on an SR5.4 (i.e it went , if somewhat lethargically, with a single 30Hp & a dead one alongside)
Best bet is probably to ask for a test run, or if the package price is good & the 20 is in decent condition but you aren't convinced by the top end, you could sell the 20 and use the profits to buy something a little (or a lot, depending on your view on it!) bigger to hang on the transom.