Having lived with a 99-740i Sport Package, model for the last 5 years I would encourage you to proceed with some caution on used purchases. Basic rules apply here, know the service history and inspect carefully. Stay away from an accident-repaired vehicle. I advised friends to consider a CPO model from a dealer with the 100k warranty; you will use the warranty on this car.
The vehicle is the top of the food chain for big luxury sedans. At 94K miles mine looks and drives like it was new. A Lexus does not hold up as well on the performance and feel but this model is no Lexus when it comes to repairs. You can expect tires to go quickly (every 25K miles), power steering hoses every two years, O2 sensors, sunroof wind noise, hood sensor and coolant level sensor to name a few. Brake rotors cannot be turned and must be replaced every other pad change. Little things can be costly, but the power train on this car has been flawless. I budget about $2K / year to keep it in top condition, and I do yearly fluid changes (brake, coolant).
I plan to drive it at least another 100K miles. There are just no other cars in its class that match the drive and performance. My only concern is the transmission life span. It is “sealed” with lifetime fluid. You will find varying opinions on if fluid changes should be attempted. If the transmission goes, the expense will be significant (mine shows no signs of wear, so far).
This car is one very safe, high performance vehicle. If you get a true sports package model beyond 99 you get stiffer, shorter springs, M parallel wheels, shadow line trim and a quicker differential, plus better seats. Happy hunting