Re: smaller sprocket on rear wheel
a lot of it depends on weight (yours and the bike's.) if you're 135lbs like me, and you're motor has a lot of torque (like mine) you can get away with a lot smaller sprocket than a guy who's 200+ with a stock motor.
eventually, a heavier guy can get to (almost) the same speed, but it just takes longer to build up the momentum.
i run a 30t, and it works great for everything, hills, etc.
ideally, you'd want a balance of speed and torque, and to do that, you'd have to experiment with different sprocket sizes, which means spending more money and time.