Something at least 50 litres. That way even on a small job it will still be fairly stable. Nothing worse than knocking it over full of slurry in customers house (everyone does it once...dont they?)
Twin motor is way to go however there are a couple that can handle it with a single vac. Having a longer hose on the machine will always be an advantage and save having to wheel the machine around too much.
Get one with a friction free floor tool. ie. the squeegee has wheels. this way it will pick up going forwards as well as backwards.
I personally use Truvox machines as the replacement motors are easy to fit and cheap.
Cant go wrong with a NUmatic either. Just get one of the bigger ones.