The second hand market in Argentina is not as extensive as you'd like. This means, IMHO:

1) To state the obvious first, if you have lots of cash you can buy almost anything you want. Although due to import tariffs, new(ish) bikes will be more expensive than elsewhere. Eg: A new F650GS costs 12k+ USD in Arg, against 8k+ in the US. Similar figures for a Transalp. And they hold their value very well. Even an early nineties Transalp will not sell for less than 3,000 U$ unless damaged.

2) There are some 350s, 400s, 600 and 650s around, sometimes at reasonable prices, although for some reason I do not understand the availability tends to be of bikes which are at least 10 yrs old. A few of them in good condition, most with high mileage or sth looking suspicious. If you are hunting for a specific model such as an Africa Twin or Dominator you will just find bikes from the few years in which they were imported, if any owner is willing to sell. Don't expect to find much outside the Honda, Suzuki, Kawasaki & Yamaha quartet plus the odd BMW. Only official dealers are Honda, Yamaha and BMW, the rest being sold by independent importers. Ok, there might be HD and Ducati dealers too, but those are too focused and outside what you are looking for, right?

3) There is a wider choice on XR models, specially 250Rs and 600Rs (without e-start) and some 200Ls and 400Rs, as they have been "fashionable" in Argentina for some time, even for road use which doesn't make much sense.

3) The NX400 Falcon is much cheaper than older imports, because it's imported from Brazil which has reduced tariffs due to Mercosur. Price new: 16,000 AR$, about 5,300 U$. Used bikes tend to show up more often and are only a few years old, as the model was introduced not so long ago. Still, expect to pay at least 3.5 - 4k USD for a good one, watch out for couriers. A much cheaper option is the brazilian made XL2xx (although much slower). Again watch out for couriers.

4) If you want to scan typical options try:

www.deremate.com
www.mercadolibre.com

Which are e-bay style auction sites. You'll need to be able to read Spanish, though, or just browse the pictures.

5) I have no idea if you can register the bike in your name being a non-resident. Sorry. I recall, however, from some other thread or search that there's at least one dealer who will offer either rentals, or sell you bikes with buy-back options for these trips.

PS: just to clarify, my signature shows Lagos, Nigeria, which is my current residence place but I'm an Argentine and go back there at least twice a year.

Hope it helps and g'luck