For the paint work, after waxing apply a silicon resign polish which seals the paintwork from the elements. Do this 2 to 3 times a year. I've used Auto Glym Super Resin polish in the past, not sure if you can get it in the USA.
For the interior, protect from bright sun light as much as possible, valet once or twice a year.
For the mechanicals serviceing by the book from a BMW dealer or independant specialist who knows what they are doing. Always keep in mind that the majority of engine, gear box etc. wear occurrs when the car is below operating temperature, i.e. in the first few miles of driving. Avoid short trips, do not warm up the car by idling it on the drive, this is the most damaging thing you can do to the engine. In winter try to garage the car so it does not get so cold. If the winters get really bad consider an engine block heater, popular in Canada and Northern states. Some taxis are know to rack up 500,000 to
1,000,000 miles before dieing, This is because they are running 24x7, so the engine is never cold.
Lastly financially plan for replacing this car once it as about
250-300,000 on the clock. What ever you do it will not last forever and get expensive to maintain. I'm still really impressed you got 243K miles and 16 years out of the last car.