If you can get cheap batteries, great!
The 4 Optima Yellow Tops were the single most expensive thing on my motorcycle. (about $600! Ouch!) That said, they have been great batteries for this project.
My cycle is a 1981. They don't do emissions testing or anything like that on them. Make sure you get a TITLE when you buy your cycle! Some states aren't too bad in the hoops to jump through to get a replacement, but others, it is literally impossible.
Insurance wasn't a big deal. I have mine through progressive. You might want to talk to an insurance agent BEFORE you buy a cycle. If you have a crotch-rocket with a tiny electric motor, insurance will still be based on that it was a sport bike with a huge engine and wreckless attitude.
Make sure to get a cycle that has enough room around the engine to mount all your batteries. You will want to make sure that it at least has dual down-pipes. If it doesn't have passenger pegs, don't get it. If it wasn't designed for suspension and brakes for two people, it's not going to suspend and brake you AND a couple hundred pounds of batteries very well.
The big mistake I made in my bike was getting a frame with a messed up wire harness. I ended up cutting the whole thing off and making my own. It works fine, but it was a lot of work and still looks like a mess.
If I was doing it again, I would spend a little more money on the frame to get one that looks nicer. I spent $100 on mine, and it had a ding in the gas tank, some surface rust, no ignition switch (not needed, but it would be nice to lock the handlebars) and a rusted switches.
I bet if I really looked around, I could have found an appropriate cycle, with a nice-looking frame and broken engine for about $600.
Glad to know that somebody else has been reading about my cycle!
-Ben
PS: learn from my mistakes and make yours even cooler!