Dude, seriously, go for the bike that has the biggest engine that you can afford......
The speed and terrain is all up to your discression.... The speed is how much you turn the throttle and the terrain is all up to where you want to go..... its all about practice!
To go with a 150cc - 200cc bike would be silly.... it would work, but you would be giving up performance. When on the road you are going to need the breaking and acceleration ability to be able to get out of sticky situations (when that driver on a cell phone pulls out or breaks hard because they are not paying attention!)..... and the bigger bikes would offer that in a much better way than the smaller bikes.