Hi, Carolyn! 
Before you buy a smartphone, be prepared that you'll have to charge it every day or every other day depending on Internet use. Also, you'll get to make substantial use of app markets to download some useful apps and make the most of your smartphone. And last but not least, you'll need once in a while to make updates of the software, which is not smth terribly intimidating. Better test one, but I think smartphones are just for everyone :)

As for an M-tel plan, they have some pretty good offers for subscribers for unlimited access to their 2G/3G network (only limited in speed), which, however, don't apply for pay-as-you-go. Indeed I think you better buy a phone in the US, but should you use it in Bulgaria, I think you'll have to decode it, which compromises its warranty. Good luck!