G'day.
Use ZF6 fluid. Its what is in it now, and since you cannot change it all at once, just use that. Plus, there is **NO** other fluid that BMW nor ZF recommends.
(and yes, people that sell other fluids will 'recommend' theirs.)
I do a 2x drain/fill every 60-70k. Drain, rough fill (use old seal, just semi-tight) then drive for 5 minutes to get it all hot, select all gears a few times. Then drain and final fill (with careful top off procedure)
The tasks you propose seem reasonable, not that difficult. There are a few threads on the GPs and controller....
For transmission parts stuff, snoop over at thectsc.com...not that they can ship to you, but some good info.
newtis is essential.
I also use some of the dealer parts websites, they can be a bit newer than realoem. bmwofsouthatlanta, getbmwparts and BMW of san francisco. But there are others I am blanking on.
136k? She'd hardly broke in!