any paving experts on here?

jos51700 said
4-5 feet. Four preferred, but a portion may have to go to five.
I was planning on a soil barrier fabric, and then a 1-2" diameter gravel back fill behind the stones, maybe eight to twelve inches from stones to soil, with a 3-4" pvc drain at the bottom of the gravel, similar to how you'd French drain a foundation.

But I have no idea how to do the base under the stones so they don't walk with water penetration and frost heaving

Sounds like you have a good plan on the back fill. Make sure before covering the rock with soil the cover it with fabric so it keeps it separated.
You will want to dig down below your normal frost line about 6 -8" and pack it with crushed gravel. Add 1" of sand on top of it. Make sure it is level, level , level. Your first row of block should be below grade and use blocks that you can pin together. do not rely on the blocks with the lip on the rear for anything over a couple of feet high. By pinning the blocks together it will keep them in place for a long time.
Here is a video that shows some of the reasons retaining wall fail

Good luck with it and post some pictures when it is done