I have paddled most of the boats mentioned in this thread. When I first was learning I tried the RPM and absolutely hated it. I switched to Fun and never really looked back. I have also paddled an EZG and it is also a great boat that actually paddles very similar to a Fun (the older generation Funs anyway). As was mentioned by someone else, a planing hull river runner like a mamba or burn would also be a good choice.
You should definitely be able to find a used Fun for $400. The EZG is a fine boat too, and you should be able to find some cheap since they are so old.