If you have very small hands and can stand on your head you can replace the hedgehog just by popping off the cover in the footwell.
As far as servicing goes I think you'd be a mug to try and extend any service intervals. My annual oil change will mean that it's only done about 5k miles but the one thing you can't really fix yourself is a worn engine so to me it's the best insurance you can get. As with any E39 you'll want to be handy with a spanner as I wouldn't want to be putting the car into a garage every time you get a little niggle.
If you can get a good one you shouldn't find any issues with the Alpina upgrades, as I said earlier it's just the regular E39 issues which are part of the fun of owning a 10 year old car.