I wouldn't worry too much about water getting in. I've got hatches on the sides of my seats which aren't waterproof, but it only takes a sponge to mop out any water that does get in. Only seems to happen when the rain is coming from a certain angle. Hmmm west coast of scotland.... quite a lot of rain.... perhaps a top spec hatch would be worthwhile
As far as reinforcement goes, I don't think you need to beef the console up with anything. However, it is worth including a few offcuts of ply in strategic places so that you can screw things in place on the inside with self tappers.
And as far as steering goes, you want a 'no feedback' (NFB) steering wheel. It shouldn't turn unless you are actually turning it. Not sure how this works if you want to keep your tiller operational though. I don't think hydraulic is needed unless you can get one really cheap, they're about £300 new