Well, as I said, I am listing these in order of perceived difficulty to me. Garrus is hard because no matter what base figure you use, you are going to need to do some converting. And Liara is difficult because she seems so simple in concept that I won't accept anything but a very good final result.
Since I know I am going to be doing a bunch of converting on Garrus, and will likely be unhappy with my first go at it, I figured I would go with the easy to convert and cheap to replace Halo Action Clix Elite. This is a fairly apt choice when you look at it from a certain perspective. Much of the armor, physiology, and posing of an elite matches Turians pretty closely. It's a soft plastic, pre-painted miniature, so cutting, gluing, and pasting should be a snap. The only problem is that I will have to scratch build a head, but that's true no matter what mini I choose for a base.