I've just bought my SH7218U for about 250USD, including shipping, comes with an extra battery, external charger for the battery, 8GB microSDHC from a fairly reputable seller from Taobao.
Do you come by to know anyone who buys/bulks from Taobao occasionally living around your area, or perhaps is there some community forums where you have a fellow-forummer acting as an agent for you? I find them much more reliable than purchasing via a random site online, and these agents will help you handle off most of your worries.
But that's just me. lol
Regarding the OS - You can't actually change the OS per say. It has to be an Android, and a version 2.2.1 (unless 2.3 is ported over successfully). What you can do, is to learn how to flash your ROM to a customized ROM using CWM. junkman written an excellent guide on doing this, which you can find on xda-developers forum. I am not allowed to post any links at the moment, so just google for "XDA SH7218U", you should see the thread from XDA result on the first result link return.
All the best, and have fun!
I am still trying to figure out the texting part. I do a lot of one handed texting so I use the 12 keys keypad alot... But not even Sogou input in working 100% fine with the keypad input.