They do in fact charge you tax. The only way around that is to have the current owner get you a temporary tag. If you're buying from a dealer, I'm not sure if they would or not.
I got my bike in Florida and should have paid taxes, but got super lucky when the girl at the counter suggested we get a temp tag in the current owners name, he was fine with it and it only cost me $12 in total if remember correctly. I rode the bike back home over the 3 days, so you'll be fine driving a car back. I've also heard of people of taking their ONT plates with them and sticking them on their newly purchased vehicle in the states and driving it back home. Of course you take your chances with that one as it's certainly not legal, at least not here. Not advocating that you do that, simply saying that some have done it.
You would need to bring proof of insurance with you into the states. Your insurance co here will need the VIN, make, model and year and value of the vehicle you're purchasing in order to give you insurance. At least that's what mine asked for.
Good luck with the purchase.