BMW

Just go shopping and find what you like. Check out some car stereo shops, and the same for rims & tires.

Best is probably a matter of opinion. It all depends on your taste and how much money you want to spend. Newer wheels from a M series car would be cool.

Find what you like. Whatever fits really. Personally, I think there are a lot of BMW wheels that look great on E39 5 series. Aftermarket wheels are hit or miss... I really like Varrstoen ES2s. Otherwise, BMW style 37, style 32, style 65, style 66, style 71 all look great. Just make sure you get the right center bore and bolt pattern, being that these are hub centric wheels. As for the stereo, I would keep the head unit stock. Maybe better speakers or a better amplifier... I'm not a fan of the stock sound system in these cars (at least in my '99). Anything other than the stock head unit doesn't look that great in these cars. Either way, it's your car. Do some research and get what you like (and of course what will work!).