With the substantial disclaimer that my only experience with Di2 so far is five minutes of playing at a bike shop, it sounds to me like the only role played by the shifter is to tell the derailleurs to go up or down, so I think as long as you match derailleurs, you can use any Di2 shifters. So I'd use XTR front and rear derailleurs, since XTR is the only Di2 set that supports a triple. For the crank, why are you asking? I haven't researched enough to choose a specific model yet, but I'd probably consider something lower tier in the Shimano MTB lineup. If you're hinting at the maximum chainring size that the mountain derailleur can handle, I assume that's an issue I'd have to consider, but I don't think I will miss a massive chainring on this kind of bike anyway.

I talked to a friend of mine who rides a Surly Crosscheck with road components earlier and I think now I'm seriously leaning toward just getting the complete Straggler for now. Obviously will have to ride it and see how I like the SRAM components.