On the up side I have access to 3D printers and a laser cutter so as far as weight is concerned it is going to be pretty low. I'm still wrapping my head around the electronics for now, apparently the level of detail I can get to is around 256 different speed variations, yay for mosfets. I might switch back to an arduino or pickaxe because of the analogue io pins.
I should have taken some pictures of the whiteboard I was nutting this out on.
Keep in mind whilst it would be nice to have it hold vertically stable, for the first prototype if it wobbles up and down a bit it isn't too detrimental.
The thought around hydrogen has been brought up many of times before, for the first prototype I'm just using what is easily accessible and available to me, it is definitely something I want to look into after the first model, along with actually making my own balloon instead of using standard rubber ones.