Be careful with speaker size. I had my super low mileage, slightly performance modified 2000 Grand Prix GTP that came with 5.25" speakers, but everyone said to get 6.5". I got the 6.5" and had to create my own mounts to get them to work with the metal hole in the door itself and still not impact the plastic/vinyl speaker grill. Sounded great when it was done, but it was some unexpected work that I hadn't accounted for at the time.
Just about any aftermarket head unit will be an improvement. With that vintage vehicle, you should just need the correct wiring harness and be set. Not sure if Crutchfield includes wiring harnesses for free still when buying car audio or not.
Look for bluetooth and Apple connectivity on the head unit.
I currently have a couple model year old Pioneer and while it sounds great and loos nice, they have this stupid dial/button/joystick thing to control most of it. Next to impossible to make any adjustments sitting still, let alone driving. Not sure about their current models. Might be worth a stop to your local Best Buy to get a look/feel for some of them and the prices.
Might be tricky to find a low cost head unit with bluetooth and Apple support. Then again, one man's cheap might not equal someone else's.
