The easiest way to get it waterproof, is to put you receiver and speedo in a waterproof box. For example those boxes in which they sell paperclips and stuff, will just cost you €/$ 1,50. Put your speedo and rx in it. Make a little hole where you want the wiring to come out. Afterwards you can close that hole with some cleaning putty. To make sure the box is really waterproof you can put an balloon around it in case in might be leaking.
As for the motor, since most BL motors don't have any holes, you can use that one without a problem. The only thing you should keep in mind is the sensor. The sensors has to stay completely dry, otherwise you're motor will malfunction of break. Or when allowed, just drive sensorless, than there is no problem whatsoever.
About the servo, you could buy a waterproof servo, but actually all I did was a line of glasstape around the opening of the servo, and some cleaning putty on the back where the screws are. And I never had any problems.
And of course after racing clean/dry the car completely. And dismount the entire car as soon as you get home again and clean every single part again.
A nice tip for that, when you get home, put you car in the over (not the microwave
) and put the oven on 50 deg. celsius. Keep the car in there for 10 minutes or so, and a lot of the parts at least are completely dry, than just clean them to be sure. And lubricate all your bearings and stuff.