Brosiden,
I've been through all this recently mate, so I know what a dilemma it can be (in fact I'm still going through it - got the boat, but still haven't got my hands on the engine I want).
Like Trelawny and Scott Wilson said, you probably need something about 3.5 - 4 meters long with a 15hp engine on the back (there's quite a large jump in weight when you move up to engine sizes above 15hp, and probably not enough 'push' for your requirements in an engine that is smaller).
People say be wary of engines on e-bay, which is fair enough, but sometimes you can get some genuinely good bargains (and the best/cheapest way to buy is often to buy the boat and the engine together - with trailer thrown in). A mate of mine got a lovely deal, on a bigger inflatable, with everything he needed thrown in for £1600.
Be warned though, you might find a boat going cheap but you will rarely find the same with a decent engine - they are like rocking horse s**t and everyone is after the better ones (you'll be in competition with me, for a start). Expect to pay upwards of a grand for a boat and engine that work well - and that's a pretty good bargain!
Mate, I work with boats, including inflatables, and I'm struggling to find what I need, so if you see a bargain, go for it.
Other people are giving their top tips, so here's two from me (especially as Scott mentioned big waves):
1, Use a kill cord, especially when you are on your own.
2, Get a capsize line attached to the side of your boat, and learn how to right the boat just in case it does go over (in the surf, in the races, etc).
Hope you find what you need.