My vote for "best bang for the buck" goes to the ebike.ca's 9 continent motor kits. Why? They bundle it with a cycle analyst, and the service is very responsive and helpful.

It's a direct drive motor, so it will add some drag, but has a higher top speed, lower price, and longer effective lifespan than the geared hubs. Direct drive hubs also offer regenerative braking, essentially extending your range a small bit as well as reducing the wear on your brake pads.

They offer it with up to an 8 speed freewheel, so assuming your dropouts are steel, you could spread the frame a bit. If not, then your still in luck, as your chain will work on 5-6-7-8 speed rear clusters. (Unless it's been on there a long time, then I'd suggest just buying a new one). Worst change would be swapping your shifters.

http://www.ebikes.ca/store/store_nc.php