Yamaha decided the XT660Z's would not sell enough in N. America to justify the expense of EPA approval, so XT's out here.

Re-read my last post, most of your questions are answered there.
X Challenge are around here and pretty cheap. Two were for sale here in San Fran Bay area last week ... around $3500 to $4000 for nice ones, low km. NOTE: Sourcing parts could be a problem. Check on USA X Challenge forums to be sure before you buy.

But many more Suzuki DR650, KLR650 and Honda XR650L for sale everyday. Parts for all in ALL larger USA towns.
Registering no problem. Local address helps but are work a rounds on this or dealer can help.

If you have a lot of time and are willing to ride highway more slowly where you cannot ride off road, then a 250 could work out. It's been done. But traffic will be traveling at 70 to 80mph ... so a 650 might be better on faster roads. Most off road riding will be easy, so a 650 will be OK for most ... unless it rains a lot, mud can slow you down! But if you go looking for more technical riding, you can find it. For that, a 250cc bike is the go, IMO.

The TAT and other Cross country dirt routes are fine riding a 650 class bike.
So up to you on which bike. For USA, I'd pick a 650. For Mexico, Cent. America and S. America, i'd pick a 250. Good luck, have fun.

Most everything you need to know is here on HUBB ... search around a bit, it's all here.