Gee, you don't want much do you
500mm is rather small and 40mph is pretty slow. Just about any decent stock boat straight out of the box will run at that speed. I'm not being rude, usually newcomers to boating especially if young want triple digit speed and 5 foot long. R2Hobbies have a fair range and if you watch their videos the boats will travel at that speed. Please, I beg of you, DON'T buy Hobbyking's F1 Tunnel. It's an absolute heap of garbage. It's that bad I've spent twice as much rebuilding it as the darned thing cost in the first place. Burnt out ESCs and motors. At least with R2Hobbies all the boats are fitted with the right gear to do the job and better. If you become a member first you get a 10% discount too.
There are plenty of boats available which will fit your needs. A bigger boat is more stable and are usually designed to take a pounding if the lake surface is choppy. Obviously the larger the boat the more power you need although I sell a 3 foot Hull which is extremely light but has the strength. It's designed by young engineer in Hong Kong and I bought one of his hulls and was so impressed I became the sole Aussie supplier. I'm actually waiting for his new hull to finish trials and I will buy a few to see how they go. This isn't an ad by the way. I have only a couple left and it looks like they're sold but check out Ebay. Try to stick to new stuff. Used boats are easily palmed off on Ebay and suffer from many hidden dangers but the new ones are OK.
Have a good look around. See what people are talking about on boating forums. You'll know when you see the one for you and when you do don't feel bad about joining a few boating forums and ask as many quiestions about the boat as possible. Good Luck and happy future boating.