I wouldn't get too hung up on the whole waterproof thing.
Just don't go driving through standing water is the simplest answer.
Even wet grass might be a problem if the electrics start to get wet but they do dry out again.
You have to ask yourself would you be that happy standing in the pouring rain running a toy car,if the answer is no then don't worry about waterproofing too much,just do like I said which is avoid standing water.
Whatever you do make sure you buy something you can get spares for,people here have bought cars and then not been able to easily get spares,it might limit your choice a bit but sticking to one of the major brands such as Tamiya might help in that respect.
Fun is the thing to bear in mind,you can have as much fun with a £100 car as you can with a £400 one and the £100 one is probably going to be cheaper to fix as long as you can find the spares.