You need need SoA to upgrade your trial account (so you won't have to start again, it also comes free with 1 month). Moria is the first expansion you have to pay for but you won't need it until you reach level 50 (unless you want the runekeeper and Warden).
It has already been said but search for the Compilation Pack that includes both games (and in my shop it was £5 cheaper 0.o)