I'll add my opinion too....don't forget, it's free...and probably worth what it cost
1) Don't think the nav is too bad TBH. I have had Destinator on a PDA for a while and find the G3 nav in our 166 keeps up a lot better. I've yet to really push it, but it recalculated a route well enough. I'm sure more modern kit is better/quicker...but it's not bad, and it's perfectly integrated of course. If it was as slick as the system in my mates IS250 I'd be delighted! You can get new maps for G3 units from Navteq. I've been fiddling about trying to get more up to date maps by using assorted downloaded Opel discs but none are working, so I'll probably have to dig in the old wallet some time. My current maps are 2001 vintage
2)DSP/premium sound is an extra, over and above the multichanger AFAIK (but I could easily be wrong!). I've also heard that the rear speaker grills can be present even if you don't have the DSP & speakers(??), so probably the safest way to check is fire up the ICS and have a quick look in the AUDIO menu. You'll soon spot the DSP and EQ options if present.
3)Dunno really, but it's probably safe to assume they are pretty good compared to conventional. My car has conventional bulbs but still has HL wash. Repairs to failed OEM Xenons are expensive apparently. Interesting discussion about high output bulbs/aftermarket Xenon conversions ongoing at the moment... Given I haven't even driven ours in the dark yet, I'm probably not best qualified to comment though
4)Wot MLC said!
5) Have the Sportronic - quite liking it, although 5 gears would be better than 4 given the nature of the 24V motor. Strongly suspect we will get another 166 before too long and it'll probably be a Super for a bit of variety!
Again, ditto to MLCs comments. May have paid a little over the going rate for ours...but it's a good one with almost every option (including the phone, which works really well) so we are happy. It really is a tough call. Don't get too hung up on low mileage (I did at first). As long as it's been properly looked after, that's more important I think. Obviously, low mileage properly looked after is good too, as long as you are ready for the big service, either with DIY or pay a good specialist to do it right.