I agree 5.8ghz is totally the wrong end of the frequency spectrum for ground vehicles.
Ideally 910mhz high powered system is much better.
For waterproofing lipo batts you can see here:
It's a long video so skip to the 24:30 mark. (basically use silicon to seal all the gaps)
Hey Dimitris - have you tried the Wifi digital system you are talking about? Sounds pretty cool but I am curious about the range and latency of it.