I have not heard of a bypass filter. What does it do exactly?
The OEM oil filter has a check valve to prevent oil from
draining out of the engine (reduces engine wear on cold
starts) and I wonder if a bypass filter would disable that
(I'm assuming it is a second oil filter). Maybe not if
you use two OEM filters.

I use OEM plugs in our 95 855 with 56K. They work just fine.
and I'm only on the second set now. Set of 5 is under $10
at Volvo part discounters (volvoparts.com, etc.).

What are the goals for the K&N? The intake will flow enough
air for the turbo model and if the goal is more power then
it won't make much difference. If you want a re-usable filter
then it works for that. OEM paper filters go about 30K, too.
I prefer oem paper filter since I believe it filters better
then K&N.

You might want to flush the tranny fluid. That can extend the
life (so I hear). I know after ours was changed the car shifts
much smoother (feels like new). Really should be done every
30K miles, or so, but Volvo says never do it (so often the
tranny has never been flushed).