The 400 series is more than ample for your needs.
and is easily put together.Which motor depends on your route, if you have a lot of stop and go a 408 will be quicker excelleration from stop but if long runs without stops a 406 will have a faster top speed. I would recommend Justin http://www.ebikes.ca/store/
for supplier as his prices are the lowest, but i don't know if import taxes apply.
I agree the Nicd seem the best choice for price and recharge cycles.If you run 48V at @ 8 Ah using Nicd 3C with a 20A controller you should get over 30 miles(I get that with this set up with hills using immediate start controller)
If you go for the 48V 20A controller do not get the large black box (the one with reverse key) as some have fets rated at 50V make sure the fets are rated over 56V which is what 48V Nicd are at full charge.
As you want a front wheel a torque arm is essential and will save you many headaches, better if you make your own.
More volts = faster top speed
More Amps = higer torque but reduces battery/bike range and you dont need this because your on the flat.
Get whichever charger is recommended with battery
Don't get the throttle with leds,it can short on full throttle
As for battery bag wait till everything is assembled as you may wish to move battery location for better weight distribution.
Did you look into motorizing your trailer instead to push you along
If you did this you could go with a much cheaper set up using SLA's and a cheaper motor.
hope this helped you dude
Not everything that can be counted counts, and not everything that counts can be counted.
- Albert Einstein