[snellemin]

If your requirement weren't so high, I would say go for it. But 45MPH should not be attempted on a cheap bike. Disk brakes at least for the front is highly recommended. A good front suspension is a must. The frame or at least the rear part of the bike should be chromoly. If not, increase the strength of the dropouts to deal with the torque.

30mph is the most I would go on a regular frame bike, with the use of torque arms at the dropouts.

There are a few motors out that are torque monsters, but I chose the Torque monster Magic Pie for the slim design.

There are many controllers out there to get and Ebay has plenty of cheapy ones that can be modded. I didn't want to do all that after my initial Ebay kit, so went for the Lyen controller. His controller can be modded and you can ask for additional features to be installed. You can also tweak his controller with the provided software.

The 15-20km range is doable on 5000mah, but not at 45mph. 7000mah is a safer bet for that range @45mph. Go slower and your range greatly increases.
I can go 5.5 miles on 1Ah at a slower speed vs 3Ah at max speed.