Thanks for the thoughts everybody. We have decided to go with the tekonsha IQ. This will then fit beside the cigarette socket on the dash. We were told that the P3 would work but was overkill as they were made for braking super heavy boats.
The P3 isn't overkill, the P3 and the IQ both do the same thing but the P3 has more diagnostic features and is regarded as the best in proportional controllers.