Just a bit of info for anyone who's thinking about a 5 series. There's a lot of info on the net, some of which is contradictory, so maybe this will help.
I started off with a Clyte 409 kit running on a 4820 controller and a Crystalyte Lithium 38v 9A battery. Despite all the talk of torque it's prettty anaemic in Malta which is all hills. I was looking for a motorbike substitute and this wasn't it.
After looking around I thought I'd go for a bigger battery and motor. Thanks tothis site, it became clear that a bigger batter and controller using the 409 hub would not be such a good idea (glue melting and magnets falling off when using high current) so the 5 series 5304 which was said to have a good combination of spped and torque was clearly the way to go.
I got a bit confused a few times as (for instance) one importer's website stated that there isn't too much difference between the 4 series and the 5 series on a bicycle seeing that a 4 series is much lighter and can achieve similar top speeds. Well to anyone else who read that, please note that this statement is complete bollocks!
The 5 series rear motor slipped between the rear dropouts easily - I had to place a spacer between the 7 speed gear cluster and the dropout as the two were pressing against each other once I tightened the bolts. A bit of swearing there as I had to dismantle everything.
I chose a 48v 35Amp controller to go with the 48v 20Ah Ping battery (Ping is one of the most helpful sellers you'll ever come across - a genuine nice guy).
Anyway, end result is fantastic and I can't see myself needing anything else. I have no felf the need to peddle up hills including some pretty long and steep one (compare this to a 4 series hub which runs out of steam as soon as a little hill can be seen). The bike actually picks up speed going uphill even though it uses 26" wheels! 50kph on a flat is easily achievable. It's basically what I wanted - a small motorbike.
Highly recommended setup.