I've never done it personally but have heard of a method of trunk banding to limit growth. Maybe someone else can give some advice on that. MM111 get very large if they are allowed to.

Here is a link that might be useful: Rootstock Spacing

I think you will need about 20 foot spacing between trees as a pure guess. It is unusual to select espalier rootstock for drought resistance but how the trees react will depend a lot on the cultivar, as you've already considered. It will also depend a lot on your skill as a pruner as far as discouraging vegetative growth in favor of fruiting.

The majority of espalier pruning is generally done during summer, which helps control vigor.

I can tell you from experience that mm111 will be uncontrollable on a trellis system. Even the lowest vigor of cultivars. I just cut my last two trees on mm111 in my orchard to make room for another trellis system. I fit 11 dwarf trees on a trellis system spaced 5 and 6' apart where those two 7 year old mm111 trees once were.

My suspected M111 didn't bear until I gave up and let it go wild, and I had it on a huge trellis. Just not enough leaf to satisfy the root was my guess.