Keep in mind there's a little bit of a placebo effect with UV sterilizers with regards to parasite control - in my opinion.
I know people swear by it - I did too - until I had an outbreak while running one anyway.
I don't know that there's any real data on hobbyist grade UV sterilizers with hobbyist configurations - and we do know that sump plumbed UV sterilizers have limited effectiveness against Ostreopsis dinos.
(that said I use a 57w on my 180g + 40g sump, and a 15w would be a good choice for you)