If you can find a place to get Smithii's from reliably, let me know, I've yet to find any place. You can always get an N. Wennerae suitable for a 12g from tampabaysaltwater.com. $50 shipped to your door.
Those two currently on ebay are N. Osteradii and are suitable for a 12g. Very similar to N. Wennerae just different colorations. The Osteradii are more speckled and not quite as bright green initially, although both species tend to turn green under aquarium specrum after a few moults. Osteradii have a purple Meral Spot while the Wennerae have a white one
As far as behavior and temperment goes, there's no difference between Wennerae and Osteradii. If color matters to you, you might want to take a gamble on a wennerae from TBSW, if not, go for the green one on ebay
Shipping livestock internationally is illegal without the appropriate permits. suffice it to say, jumping through all the hoops to get it done would be not anywhere near worth it for a single mantis shrimp.
No, not at all, they're a very very reputable company. There is a remote possibility that you'll get a pseudosquilla cilliata instead of an N wennerae as P cilliata are also found in the carribean (although wennerae are MUCH more common in florida/keys). Either can do just fine in your tank though