Theres lots of good aircraft for FS98 just because its been out so long. This is still the prefered simulator of a lot of diehard FS fans because there was more focus on flight dynamics than scenery. I personally have Fs2004 and recommend it, however you'll need a pc upgrade everytime you want a new version of FS. DO NOT GET FS2000/2002! These we're possibly the worst versions ever made and 2004 runs better/has major improvements than both of them. 2004 runs fine on my pc, but its also a custom built one with a P4 2.4 gig, 80 gig hd, 512 ram, 128MB NVidia GEforce 4 video card (theres better ones out there now but a year ago this was decent and still is for almost everything). I'd also recommend getting a force feedback joystick, this ads a lot to the flying.
You have to be a member at flightsim.com, but registration is free though logging on to get into the file libraries can be a problem since they have a limited amount of bandwidth.
Avsim is decent, a bit slower download rate than flightsim, but no messing around with login. selection is decent but not the greatist.
There might also some good cheap payware you could find on ebay. stuff that originaly sold for 30 bucks you can prob get for 5.