'86 4000cS Quattro

[DK]

I would keep it, as this car should last you for quite a while, if you
change the oil frequently and do the timing belt on time and take care of
it.
The instrument cluster usually like to seperate, at least all the 4000's I
have seen they have been loose with broken plastic tabs. Its a simple $3
fix, getting some frame brackets and some glue, and you should be good to
go. A/C - not sure where you live and how much you need it, alsong as you
are not in arizona, you should be fine. You have a sunroof. I removed all my
A/C stuff, as the system was dry, unknown condition, and it usually takes
quite some troubleshooting and $$ to fix it.
All wheel drive doesn't work? Sure it does, it is always on. You are talking
about the knob in the center console - that is not to switch it from 2wheel
to 4 wheel drive, its to lock your center and rear differential. I don't use
it too often, unless its really deep snow, ice etc out. What most likely is
broken is a vacuum connector going to it, its disconnected or just stuck
(maybe rust or hasnt been used for so long) so it doesnt move anymore,
meaning some TLC and some extra help with some lubricant could get it
working again. Neverthless, you still have a all wheel drive system that is
fully operational, unless the drive shaft is gone.
trouble with going into second gear? Is the whole shifter a bit slopy? could
be a worn bushing, the levers need some adjustment, the transmission or
engine mounts are loose/ worn out, or it could be your clutch hydraulics not
working all the way they should.
But over all, fixing those small things shouldnt take too much of an effort.
I am not sure how experienced or handy you are, but those are easy fixes
that you could do on a saturday afternoon.

If you are happy with the gas milage, you should be good to go. Aslong as
there are no major issues, keep it as long as you can and enjoy it in the
winter.
Feel free to ask for help,

Dave