Sorry to hear it keep costing you money.
A 99 GS is a good one too. You'll soon learn about the many Regal quirks.
Most of the things I found were
Worlds most often broken cupholders, and most expensive to replace
Wheel bearings often go like the 00+ Bonnevilles
Seat heaters that usually fail like the 00-02 Bonnevilles
Dogbone mounts to be in the way
Tighter engine bay
Horrible to put headers on.
Same cable driven window regulator issues as 00+ Bonnevilles.
No stinkin gas cap holder on the tank door.
The good stuff:
Every mod for a Grand Prix fits easily.
SLP rear bumper fascia'* will allow you dual GP exhaust
No stock spoiler leaves it open for cool stuff.
Great turning radius
GMPP sway bar packages
Upgraded lateral and drag links
Ability to add the GP hud (I've done 5 so far)
Might be roomier than the Bonneville of any year.
Strut tower bars from Impala'* fit nicely.
Performance struts and springs to lower if that'* your thing, or firm it up more.
Larger rear seat passthrough or 60/40 fold downs.
00 Regal GS: All kinds of cool stuff.