The 528 does have a very reliable track record (although it was actually the 530's engine that got on Ward's 10 best). At 90k miles, you should have the cooling system overhauled before trouble does happen with it. That means water pump, thermostat, radiator, overflow tank, and associated hoses. This stuff usually deteriorates around 100k and will be your major focus. Other things to look out for at that age are front thrust arm bushings, wheel bearings, and shocks/struts. There'll be other quirks, but those will be the major things. It's not unusual for an E39 5 series to run well towards 200k, so you really don't have too much to worry about, as compared to other vehicles. But like Numark318i said, preventative maintenance is the key. Reading up on internet forum boards will keep you well informed . . it's a lot easier these days to maintain a car, back decades ago, we didn't have the internet
But there are two things I see that you might want to think about. First, it sounds like your repairs are being done by the dealership. With no warranty left, there really isn't a need for that. Ask around your area and find a reputable independent repair shop. They'll charge you much less than a dealer will, and the 528 has been around long enough that there really aren't any mysteries about servicing them anymore. Also, if you hang around BMW forum boards enough, you'll pick up on things to look out for, and how to fix some of these things on your own, save some $$. Just don't get freaked out; forums typically voice a lot of problems more than praise for the cars, but that doesn't mean all that stuff will actually happen to yours.
Second, if you're planning to trade or sell, remember that resale value can only go lower as time goes by. This factor, rather than maintenance costs, should be weighed in against getting a new car. If you opt to trade up, a private sale is your best bet; a dealership will not give you much trade-in allowance for an E39, since they can't resell it anymore as a CPO.