Welcome to the forum - you've got a great car there, but I may be a bit biased
When you say upgrade the stereo, there are a lot of options - adding an Intravee will allow iPod/iPhone connectivity, speakers can be added to the doors (see PerryGunn's timeline for a how to with his Alpina) plus there are threads on here detailing anything from amp upgrades to tablet integrations. Depends on how far from stock you want to travel ...