Put it this way, given we only had two dry weekends out of six for the national series this year... you want to ask us UK guys
Personally... given I use an MRT speedo (100% FULLY waterproof), the only protection I use is on the reciver. I've moved to using self-amalgamating tape to wrap it up completely. And that's about it for waterproofing... had no problems at all. Just make sure after the meeting you check all your bearings, and keep them well oiled during. I did occasionlly run some protection over the motor endbell (used a motor tube with holes drilled in the end for ventalation), but never really noticed a difference with it on or off, and after one of my motor brush shunts melted itself to the cover I haven't run it since.
Tyres... only option I've run is the Pitsumi D20 Wets (preferably in the narrow, 20mm option). Just use the std foam insert (just remember to squeeze out any water before the start of each run
Have heard good things about Medial Pro wets, and the new Intermediate Pit wet, but not having run them, can't comment.
Setup wise... you'll loose steering, so make adjustments to your chassis to give it more steering (softer front springs, more rear droop). Also raise your ride hight if theres a lot of standing water... help avoid aquaplaning.
You'll need to adapt your drving style too, just keep it smooth and tight.