Would a 98 550 w/3000 miles have issues?

Lot's of questions. Much depends on the previous owner.

1)yes, I think 500 miles a year certainly qualifies it as sitting too much

2)I don't think the low miles should be as much a factor in the purchase price as the documented care of the previous owner. For the same money I'd rather have a 550 with 20,000 miles on the clock and perfect records than a 1000 mile 550 with poor records.

3)You will need a very good PPI to guage exactly what needs to be done. If the PPI passes with flying colors and you buy it, still expect a few items to reveal themselves as the miles add up.

Good luck and happy hunting!