The BMW e39 (5 series '97-'03) is a very reliable car. Not too many things go wrong with them. At your mileage of 44k, I'd say you may be looking at replacing the water pump, front thrust arm bushings, and maybe the front struts within the next 15-30k. If not abused, the clutch can last as long as 100k, maybe beyond.
I bought my '01 530i last July with 97k miles, and since then, I've had very few things go wrong. As far as repairs, I had to change out the exhaust cam shaft sensor ($45), Water pump ($62), front sway bar links ($30), and thrust arms/bushings ($105). The front struts were showing their age, so I got Bilsteins ($306). The 3rd brake light bulb socket mysteriously melted, so I got a new one for $11. Then, as part of the 100k maintenance schedule, I replaced my pre-cat O2 sensors ($172). I've done other stuff electively, but as far as mandatory repairs, it totals about $700 for the year I've had the car.
That's not bad at all, considering its mileage. Now at 108k, it starts and runs perfect every time, everyday. Every feature and option it came with works as if brand new- there is nothing inoperable on this car. With your car being a year younger and less than half the miles, I'd say your maintenance costs will be quite lower. That is, if you do your own work. At around $100, the e39 Bentley manual is a great investment and will help you more than any other resource around. A great site for info & tips on the e39 5 series is bmwtips.com. Hope this helps