1) Not sure about the V8's -- I'd say > 200k with them -- but I know that the inline sixes are usually good for 300 or 400k
2) don't know - doesn't the car already have an IR keyless entry system?
3) I, personally, would stay away from HID, just because of what Daniel Stern says about it.
4) Call leatherique or colorplus. My leatherique results on the entire car were great with the exception of the driver's seat (had to crack fill the sucker to death, and the dye doesn't seem to adhere well to it). For the DS armrest, it should be an easy hour or two job.
5/6) don't know
7) I'd have a mechanic who knows these cars check the system out. If the microfilter's in need of replacement, the issue will more regard actual air thoroughput, not whether said air is cold enough. Be forewarned - A/C repairs on these cars aren't cheap.
If I were you, I'd get one of the manual 535s that're floating around on the board right now. It'll be a lot like your current car and maintenance will be a little more manageable (so they say).
best, whit