I agree with the idea of buying the 10-22 (perhaps looking for a used one) and selling it when you upgrade. If it'll take you a year or two to upgrade then that's one whole year that you'll be missing out on shooting UWA. Though in sunset shots @ 10mm the sun will pretty much be reduced to a tiny speck.
I sold mine within a few hours after posting it on CL (something I regret). It shouldn't be a hard lens to sell if you take care of it.

