Those motors are about right, I've been running Novak Ballistic 7.5 in my 2wd and 6.5 in the 4wd and if anything they are a little much for Whitcliffe as the straight is quite short, if you were to go the 8.5/7.5 route you might find yourself wanting a little more at larger tracks. Though I'm really liking the timing advance the newer ESC's have which lets you run a tamer motor without sacrificing top speed, I think the SP ESC supports this though I'm not 100% sure it works the same way as the Kinetic I use.
It's hard to beat the value of the B4.1 RTR, everything you get in the kit would be more than up to the task of learing to race
Then if you were to get yourself a shiny B4.1 FT you'd be able to share spares, as anything that might break is identical on either car.
You could always let your son build the FT if he want's to build a car
