I am using Race Face cranks with 46-36-26 and 11-34 on the back. The crank length also plays a role in the leverage you can bring to bear...Long gives you more leverage but the length should still take into acount the length of your legs. I use 175mm on all my bikes. My touring bike has 26" wheels so please take into account that my gear inches are smaller than otherwise with 700c's. My lowest gear is around 21 gear inches and the highest about 108 gear inches.
As has been echoed before on this forum, not many tourers complain about having a gear too low. Many have said they are down at 18 gear inches and some even lower.