Questions about motorbike travel in America

[JayCee]

Hey

I'm thinking about heading over to the states (I'm British) buying a bike and riding south till my cash runs out. Hopefully I'll get too see some of South America before I get too broke. I've got a few questions tho

Can I get some more details on taxes?
I know some states don't charge tax when buying a bike, does this tax get added on to used bike and private sales or just new bikes?

How much money would be "enough"?
I was thinking of just getting a used cheap small single (good fuel economy and I like smaller engine bikes) Maybe something like a Suzuki TU250X with not too many miles on it. I was hoping to spend around £2000 buying the bike ($3300). Would this be a sensible amount?
I'm only going to have £11000-12000 ($18000-20000) for the whole trip including visa, flights, bike, insurance and day to day living. That’s why I'm not setting too much of a destination as I want an adventure no a race against my money.

Is Wild camping legal in the US?
I'm assuming this will be a state by state thing. In England it’s illegal as all land is owned by someone Scotlands a little different. For the most part as long as you don’t make a nuisance of yourself and keep it low key and out of the way the law is largely overlooked. Are there any laws about pitching a tent/bivvy in a secluded spot? Could anyone point me towards any sites where the laws are explained?

What time of year should I set out?
I was thinking start somewhere in the Northern USA and head South but I would like to avoid the snow in the north but avoid any crazy hot weather in the south (also hurricanes although I plan to be further west). When would be a good time to head out, I’m hoping I’ll be able to squeeze 6 months out of my money.

Any other tips or info would be welcome

Cheers

Jay