I think your shopping list makes sense, with the possible exception of the dual extrusion upgrade. I have a beta dual extrusion, and find I don't do much dual extrudin' except for test prints. It is significantly a bigger hassle to get set up and dialed in, and you can do a hell of a lot with a single extruder, especially with Simplify3D's support generation. I would recommend you get Simplify3D, by the way; although it's entirely optional, and has its own share of bugs and exasperating flaws, the support generation alone is worth it. Dual extrusion is mostly useful for dissolvable support on otherwise "impossible" prints, but those are edge cases and pretty rare. Also useful, I guess, for two-color parts, but I've become a big fan of paint myself.
The v4 extruder, on the other hand, is a solid upgrade.
But, you know, don't let me dissuade you or anything. After all, I have a dual v4 M2 myself
