Someone will need to correct me if I'm wrong, but I believe you can purchase from the Larian store regardless of where you are. You might get nicked with a conversion fee from your bank for the purchase though, as I know BoA has some asinine charges for what it considers "foreign purchases".
Personally, I felt the little extra was well worth getting a digital copy in addition to my original, especially as it was remastered(that and I don't mind helping out Larian a bit). After conversion you're looking at about $28 for the expansion by itself. Only about $12 more to get the whole deal on Steam.
Sorry, not trying to convince you either way, just want to lay out your options.
