Start with a tune up. I would do a chip ,cold air kit and exhaust . Get a scanmaster so you can see whats going on.Before you start doing any mods How many miles are on the car? You might need some suspension parts or engine parts like a timing chain if the miles are up there. Post some pics
Chip for sure, scanmaster to get everything in tune, and a new fuel pump if it's the original with a hot-wire kit. And as always recommended with a new Turbo Buick a spring cleaning - http://www.gnttype.org/maint/basics.html
Start with a good clean up and tune. It's all up to you with mods. The sky is the limit. Since it's all original, I would do simple mods so you can return to stock anytime and keep the car as reliable as possible.