So a good friend of mine just bought a '98 528i (somewhat on a whim). He is not mechanically inclined. I went with him to look at the car, but since I have very little BMW knowledge and no chance to research, I couldn't do much other than run the standard "make sure everything works right" pre-buying checklist. Now I need to know what to tell him to do next.
The car is a non-sport package with 75k miles. It's a stick. It drives very nicely and has apparently been well-taken care of (immaculate underhood, fluids fresh and up to level, four matching Yokohamas, clean carfax) but did not come with any maintenance records.
I'm aware that BMW treated cooling systems as wear items in this era, but what specifically should he think about replacing?
Also, the car has the audiophile package with the huge, non-standard shaped stereo deck. Is there an aftermarket kit that will allow a regular DIN receiver to plug in and still use the factory amps and CD changer?
Can't offer that much in the way of advice but stick is good - the autotragics on these also have a reputation of being a consumable.
hows the cupholder looking? 
I can tell from here the cupholder is broken 
Actually, the ones in the front are fine. Because they took them out of the back, apparently. 
I think it's recommended replacing the water pump and radiator at around 80,000 miles, and while you're at it replace the various hoses since they're disconnected anyway. The radiator has plastic tanks and the pump has a plastic impeller that eventually crack. There are a bunch of e39-specific web boards out there, a couple are Bimmerboards and Roadfly but there are others.
In addition to the rad, water pump, and hoses, you should also do the expansion tank, pressure cap, bleed screws (there's a brass update, since the plastic ones fail a lot), and the fan clutch. As to the last item, while it might seem like overkill, I've seen several pictures of what happens when the fan clutch fails: it locks up, overspeeds the plastic fan, and the blades fly off and trash anything nearby, including punching a couple of small holes in the hood in one instance.
The crankcase ventilation valve and associated hoses is also a must-do at this point. Failure can lead to oil being sucked into the cylinders and hydrolocking the engine. This most often fails in cold weather, especially if significant condensation had built up from multiple short trips.
