As others have said, I think this is your problem. I use a cheapo bike stand (£30 from Halfords) to wash mine right side up.
There will be holes into the frame for cables and hoses around the motor and water would be getting in through them.
The Sellaronda in the Dolomites, one of the best days out on the bike I've had. I did the anticlockwise with the climbing, but it looks pretty much the same as the clockwise.
At your budget you are really only going to get a base model bike if you buy new. EMTBs are expensive.
It's hard to say buy this or that because of the weird position the bike industry is regarding stock and also personal preference plays a huge part.
If I were you, I'd work out what size bike...
There are quite a few companies offering good hardtails for that kind of money. Stock is often an issue though.
A quick google turned up a 19" (medium?) Cube in Colchester. linky
If you have the cash on hand and are not looking for a specific bike, you should be able to find a decent hardtail...
I've watched most of this now and I think he has the wrong tyres and head angle and pretty much everything else apart from the huge balls required to do this.