My 98 V70 did not like NGK spark plugs too good, it seems to like OEM plugs the best. I replace my plugs every 20K, and I also replace rotor, cap and plug wires every 60K. When I took my rotor cap off @ 60K it was in pretty bad condition, terminals were very charred, rotor button did not look too good either. It's about $100 for rotor, cap and wires. After you replace all those items along with spark plugs, engine sounds like it's brand new.

As far as timing belt goes, replace at recommended intervals. Having heads rebuild is just waaay to expensive to risk this. Think about it: It can easily cost $2500 to rebuild the heads after the timing belt breaks. That pays for ten timing belt changes (most independents can do it for around $250) By the time you are done with ten timing belt changes your V70 will have 700,000 miles on it (do V70s last that long?) Have them replace the serpentine belt while they are at it, they take it off for timing belt replacement anyway. Good luck with your maintanence projects, have a great 4th of July.