Hi Damon. I do not know much about SSEi'* other than they require premium fuel. However the Bonnevilles in general tend to really get attached to ya! I have owned this body style (1990) since 1995 and still love em. I still miss my first H-body (Olds 88 Royale)
SSEi'* are the top line Bonneville. You won't find more knowledge anywhere about them than here. There are more than several accomplished gearheads who know these things in and out.
Welcome and hope you get to own one! And just on the surface, $4k seems really good for a well kept 98. Local dealer here has a 98, with 110k for 5100. Just to give you comparison.
Edited to add, SSEi are top line for 1998 at least, someone decided to throw a GXP trim out there I think in 2004