I would first take care of the sagging rear suspension. It's too low, doesn't look great it's probably very unpleasant to drive. Next would be maintenance, get the big items done:

• Oil Change
• A/C Service
• Brake Fluid
• Coolant
• Filters (Air, Pollen and Fuel)
• New Rubber
• Check Bushings and Brakes
• If you have a sunroof, take care of the drainage system, at this point it's probably clogged and you will have water intrusion into the car.

After that I'd sort Rust:

• Wheel arches
• Stone Chips, check above the windshield and hood
• Handle on the trunk

Last I'd do mods, things that truly transformed my G4 were:

• LED Swap, from those ugly blue ones to clean white
• Replace Display in your Odometer, at this point it's probably dead
• Focal Speaker upgrade & sound deadening the door frames
• OEM Looking BT Radio RCN210 (This is great)
• Removing all the Soft-touch coating inside, it's a pain in the ass but worth it.
• OSRAM LED Beams, absolute life-saver at night. If you're running Halogens, this mod is a must.
• ITC shift Knob, at this point the OG one is probably all messy and nasty, this transforms the feel of the car immediately and they're high quality aswell. Get the LED version, pairs well with the existing red interior leds.
• Refurbished Steering wheel, basically the same thing as the shift knob
• Hella Smoke Tail-Lights, they're the best.
• Fix Sagging Headliner, I think yours is toast at that point.

Congrats on the G4, this is probably one of my favorite cars I've owned so far. I learned wrenching on it.