Neil I wouldn't get too hung up on HP ratings. A prowler is what it is - a basic entry-level TM and it is also a compromise, which shouldn't be forgotten.
If you're stepping up from portable electric machinery to the prowler you'll be more than happy. The main factor is the way a positive displacement blower is so different from how electric motors work. Heat isn't such a big deal, it'll no doubt be a slightly different grade of hot compared to a slide-in 18HP unit but still hot nonetheless compared to a porty, even with a heat exchanger!
Having never used a prowler I still can't get my head round the fact it doesn't use a waste tank but I'll have to take peoples' word for it that it works, after all nobody has said it's a problem! I just can't compute how it is so bad for any other TM's blower to have water or foam going through it but the prowler's waste passes through its blower by design.
If you get the machine you'll certainly have top-notch advice on hand from JK should you need it, which is worth its weight in gold.
I would get an experienced TMer to go and view the machine with you if you get serious about it. If the seller can't/won't demo it for you running and working properly, walk away.