My car is 80 miles short of 130K, I got it at 124,000 over a year ago. While it's not a huge amount of mileage in such time, anyone who knows my driving style knows any normal car would probably have died by now. Mine just keeps going. The only problems I've had are generally minor things, but being a beemer they are a bit pricey to fix.
I've never heard a bad thing about the 5 series, what year are we talking about?
Servicing, get an independant to do it and you should be okay, BMW will charge you through the roof. I know Paul (scania) swears by them but I just cant justify paying double the price for an oil change. You also with bmw have inspection intervals, again too expensive for my budget but if you can afford it.

My 3 series gets about 38mpg if I really am careful, I'd imagine the 5 gets a little less maybe 30 if your careful.
In reality its a love hate thing with me, I love the car, every time I drive it I just know it'll do what I want it to. But I hate it when it goes wrong, the costs of fixing it just make me want to sell it. But then once its back and fixed I'm in love with it again.
Just make sure if you buy one it has a service history if the mileage is higher.