If they say you need 98SE then you will.
It is still possible to buy 98SE upgrades but you would have to search about.
google.co.uk would be a good start.
If you use an upgrade pack it should be quite easy, if you just bought a straight new package of 98SE it might be a little harder to do, but it would probably be better in the end.
Either way not really differcult and if you did have a problem, well theres an awful of people on this forum that will guide you through!