I agree with your assessment that buying a bike in Brazil & riding it to NY is a nightmare waiting to happen. Registration & licensing issues come to mind.
Who am I kidding. Actually, finding a bike would be a nightmare. You'd be stepping off a plane in a foreign country, probably can't speak the language, have no idea where anything is located, and your only way to get around is by taxi. Assuming you're fortunate enough to locate a motorcycle at a price you can live with (this will likely take a week), then you'll partake of the funds transfer. The registration/licensing nightmare hasn't even started. And you have no idea what types of problems you'll encounter once you cross your first border as a US resident driving a non-US registered vehicle. Does this sound like something you want to undertake?
Your "bike dollar" goes a lot further in the good ol US of A. More selection and probably about 1/2 the cost of what the same bike would cost in most of South America.
I just saw a KLR650 in todays Craigslist Minneapolis 8700 miles, appears excellent, and a price of $2500. Undoubtedly this would need another $1000 or more in upgrades before hitting the road on this type of trip, but I'm certain everyday bike buys we take for granted in the states are nonexistent in South America.
I'm inclined to think a persons first adventure trip should be undertaken with a reasonably inexpensive (but reliable) motorcycle.
Obviously, if one is on an inexpensive motorcycle, it wouldn't kill them if they chose to abandon (or sell for an obscenely low price). Not certain, but I think many gringos ride from the US to Rio (or thereabouts) and (illegally) dispose of the bike.
Of course, this is strictly my opinion. As with all opinions, weigh accordingly and decide whats best for you.