Getting started I'd look for a well set up 650 thumper, brand not important, as long as it's in good shape. It will be cheap to buy, cheap to own, and easy to sell when, and if, you feel the need. Keep in mind most adventure bikes will need a bit of work to get them just right for your needs. It's part of the fun.
Air heads are pretty cool but I believe you need a certain mindset to own one. I may some day buy one but it will not be my only bike. If you are serious about owning I'd strongly suggest doing some in depth research first.