The best place to start on anything that you've never rebuilt before is a complete, assembled engine. Almost all you can mess up then if you pay attention to where parts go is putting the new piston ring in the wrong way when you slide the cylinder back over the piston. And if someone is selling a disassembled bike with all new parts that he didn't finish, 9 times out of 10, he found something wrong and doesn't want to deal with it. If he doesn't want to deal with it, often times, you won't want to either. Just something to keep in mind.