Re: New and confused
The biggest advantage of using a mountain bike over a cruiser, in my opinion, is that the mountain bike comes with front and rear brakes and the cruiser usually will not. For a vehicle going near 30mph, you will want more than coaster brakes. That said, all my motor bikes have been built on cruiser frames, I just added brakes where needed.
The first "add on" and the only one I strongly recommend for a new builder is a direct to hub drive system for the sprocket on the rear tire. When you pick out a bike and get the kit, invest another $50 or so in a rear sprocket adapter and gear and throw out the "rag joint" sprocket that comes with the kit. Yes, the rag joint DOES work and many people have used them successfully. But this is the weakest link in the original kits (again, my opinion) since they drive through the spokes. Most of my motorbikes started with the rag joint, all of them have been converted to a direct to hub sprocket system of some sort.
Beyond that though, the basic kit is a good starting point. Go with that first and only modify things as you feel the need. But fair warning...it seems like most of us DO feel the need, if only because it adds to the fun of tinkering with these things. When you do get to the point where you want to hop up your kit, this forum has many threads on how to do that.