Re: First build, NEED HELP!
It sounds to me like you wouldn't be happy with a beach cruiser since you fancy doing some off road riding. Beach cruisers have a longer wheel base, for a more comfortable ride, but steer and respond slower. I think a steel mountain bike from the late 90s, with a diamond shaped frame would work well for you. Frames with the more modern sloping top tube have less room for an engine, and usually will not accomodate a four stroke.
I also think a four stroke would be a better motor for you. It requires no clutch adjustment and less tinkering. In general, four strokes cost more and don't have as much peak HP as a 2 stroke but have better torque, translating to good off road traction.
If you post some questions in the four stroke engine subforum on this site, you will get plenty of enthusiastic help and advice.
“You're only young once, but you can be immature as long as you like" -Unknown