Ooooh! Well done seank
1. Yes some form of tap, triggers are just about the worst way to control the flow, of all the different things I've used I like the aqua-tap the best - they do save lots of water.
2. Hot water is nice but not essential - it's best advantages of hot is that all the hoses stay supple and easy to use, if you fit an immersion element you'll keep the van from freezing overnight
3. Having used vision and gg4 I will say there is a difference, it is one of the hot topics that divide the forum, I suggest you read some threads or simply buy some and make your own mind up.
Good luck with the change over, as with everything you'll have good and bad days, any problems this forum "should" help you, well some on here will
Darran