Hello Justin welcome to motorbicycling.com glad you joined us.
Your motorized bicycle should be able to haul you around just fine with the stock gearing (44t rear sprocket). I would recommend the 66cc over the smaller 48 and 50cc motors. I weigh close to what you weigh and running a 44t sprocket going up a 9,000ft mountain in less then 20 miles was no problem for me and my motor.
I used to commute daily for years on my motorized bicycle, in fact my first motor I ever purchased back in 05 is still running. I have yet to have one single China Girl motor fail on me in 7 years of running them and I have thousands of miles on all of them.
I would be a little worried about buying a used motor if reliability is your concern. You really don't know how well or what the previous owner treated the motor.