Andy posted while I was typing - I'm not sure it's still available but the Magnum would be a very good starter (providing you can lift it)
Some good advice here already Matt.
Like Andy, I don't want to start an argument either
Every different type of boat or size of engine has it's fans.
Have a look at the latest OMRA results to see what racers are using -
http://www.omra-uk.org/resultsaadhove.html The main thngs you need when starting out is -
something straightforward to set up (the more adjustments you have, the harder it is to get right).
something predictable (in racing, you need to know exactly what the boat is going to do in any given circumstance)
something easy to maintain (fibreglass [GRP] is cheaper and easier than kevlar, wood is lighter but not as strong as GRP)
something cheap(ish) (you want quality over quantity - a cheap reliable engine is a better start than a tuned racer)
With that in mind, I would suggest an 'A' class, up to 7.5cc (.46cu in) in a fairly robust hull such as a Crusader, submerged drive - which will give you slightly less speed but more precise handling, and a solid shaft, 2 channel radio for now (you may want to upgrade to a remote mixture control later on).
The engine can be anything, but CMB are the strongest (also the most expensive). Picco, OPS, Rossi, OS are next in line then SC, MDS, Irvine, Just Engines, etc are the slowest and cheapest.
IF you can get a GOOD secondhand one, then great, you can save money and work. Unfortunately, they rarely come up and, as has been pointed out, there is a LOT of rubbish out there - especially on fleabay, and the LAST thing you need is to try and sort out someone else's basic building mistakes when you should be racing. It will benefit you more in the long run if you put your own boat together as you will know how everything fits (or even why it has come loose!).
The OMRA site (link earlier) has a list of suppliers on the "Links" page as does the main Mayhem site. You could always try a "Wanted" advert on either of the sites. To give you a vague budget figure, a good A class racing boat, well put together and with average use should be around £250 to £300 (without radio). That will include hull, fittings, tank, engine and pipe.
Good luck
Danny