I would suggest a steel frame bike for durability as the weight of the motor and battery etc. will be a great strain on any aluminum build bike.
If you are worried about the max gear sprocket speeds, you can get a front motor and not have to worry about the back. If you plan to have your battery on the back then choosing a front motor will significantly improve your handling and weight distribution. Its upto you.
I have both front and rear, my rear is in need of a fix at the moment so just running the front motor I find is actually better then when I was just running the rear.
My forks are steel and they are suspension forks, I find when hitting a dip at speed they bottom out so if you want suspension forks then I suggest getting some that are 1. Steel and 2. Have greater travel than 20-40mm like my pieces of crap haha
You can pick up a steel frame MTB in your size for around $100 at your standard 'Wal mart' or similar place.
Anywho its really personal choice. I'm sure others have some opinions and ups and downs to add
cheers