The stock mk96 puts out about 430 fps, so you may not even need to upgrade the spring - hell, I downgraded mine.
In order of importance, I would recommend from fuzzy's list first #1, 2, and 3, though I would switch g&g for a guarder clear, then in order, #6, 4, 7, and then 5 is absolutely last. The z-trig is definitely worth it. The trigger pull is gorgeous on that thing.
Another anecdote, I prefer the PDI chamber, the adjustment for side-to-side stuff is very helpful, especially if you cant your guns ever so slightly.