I'm surprised that Pete didn't point out that the freeware version of AD2000 is posted right here. http://www.simviation.com/fsdesign_aircraft1.htm
I haven't tried it & understand the principles are entirely different to both Gmax & FSDS.
I *believe* that this freeware version does not support FS98
I'm not sure that AF99 is still available
It isn't anymore - any copies you see are either in the bargain or used software bins.