It is unlikely that you will be able to fit a later model passenger seat into your '91 without some major modifications.
Stock wheels with stock tire sizes will give you the best ride quality. Going from stock 15's to 16's won't make as much of a difference in terms of ride comfort, but from 15 to 17+ will (due to the thinning of the sidewall of the tire as you move up in size and to more tire contact with the road if you're going with a wider wheel/tire). Just keep in mind that as you move up in wheel/tire size/width, you sacrifice ride quality.
Last edited by BR1AN; 05-29-04 at 10:27 PM.