RNLI sell a Severn in RTR form. It's nice but really cries out for detailing and replacement of the motor & radio gear. I've seen a couple of these done up and they look the part. I also replaced the motors & radio in one for my ex-brother in law and it was pretty good. He never bothered sailing it

but I tested it before handing it over and with twin 380's it got up on the plane and looked part at the lake.
We found a 6V gell cell would nearly fit in the supplied battery compartment. The model is held together with screws so easy to get into. You need a bit of work with car body filler to fit new shafts but nothing worse than a kit will need.
I know it's not as good as a kit you made yourself but with a bit of work you can be on the water quickoy while you build the kit. Budget just over £100 all in and you'll be OK.
Phil
http://philsworkbench.blogspot.com