Most of the carriers that are selling it cheaper than the $349 price require it to be active on their service for a particular length of time. AT&T Prepaid is six months. I think Simple Mobile (and Straight Talk, Tracfone, Net10, and the rest of the America Móvil brands) is a year. You'd probably be better off getting the unlocked one from Apple.
In terms of support, the A1662 model, which is sold unlocked at Apple Stores as the SIM-free model and is also the AT&T, T-Mobile, and Verizon model, seems to not support 2600MHz for LTE (this is the one sold on Simple Mobile, by the way). If you want that, you'd need the Sprint model, A1723, which is also the "rest of the world except China" model:
https://www.apple.com/iphone/LTE/#iphone-se
I haven't dealt with it or paid attention to Sprint much, but you
might have some luck buying a Sprint one at the Apple Store for full price, which should have it unlocked if that's cheaper than buying it in Sweden.