This is what you will need to do for preventative maintenance, in priority order:
1. Replace your expansion tank and thermostat. Also consider belts and hoses. The cooling system is the one critical system to engine survival. Some part WILL fail at 100K+. Best to change them all but you've already got the WP and radiator covered.
2. Replace your Vanos seals. On the 2.5, you need every bit of torque available. This will restore your engine's variable timing back to spec. This is the single best improvement to engine performance available. Do it yourself for $100.
3. Learn to DIY. An e39 is relatively inexpensive to buy but can be expensive to maintain. Sending the car out for service can get VERY EXPENSIVE. This forum has procedures for virtually everything that can go wrong with an e39. DIY will cut your maintenance bill by 2/3. It will allow you to buy the tools you need and do the maintenance you would like to do (e.g. suspension) but can't afford to do.
4. Read this forum. There is a wealth of knowledge and exerience available to eliminate surprises and fix problems on your e39. Learn to search the forum for specific issues. And pay it forward by posting your issues and resolutions.
Enjoy the new wheels!