In relation to some of Styria's comments, yes, a 6.3 can be a right pain in the arse. The car was not designed for the M100 engine, and as such, everything in the way of ancilliaries are placed in compromise positions.If you can do the basic servicing yourself, it is OK, but through a dealership it is expensive no doubt. As they say, for a 6.3, it is 6.3 times the cost.
The fuel injection is a wonderful piece of engineering. The problem is that everyone expects them to work perfectly - even when they are 37 years old. That is just not possible without regular maintenance - like any other component. The FI works pretty hard all the time, and if oil levels are unchecked, there can be some serious problems. Cams also wear, and are almost impossible to find now. I forget exactly what a new injection pump costs, and yes, they are still available, but I think it is in the range of EUR 18,000 (A$28,500) so I don't really expect anyone is going to go out and buy one any time soon. This was one of the things I had reconditioned when I first bought the car, and while it was not cheap, it was also not exhorbitant - Less than $1500 with some additional work done, and this was with them removing and reinstalling it. Finding a specialist is the tricky bit, but they are still around. The second major area is the air suspension, but again, if you can do the work yourself, should be able to replace the 3 valves for less around $1,000 with reconditioned units. Yes, they are a pain to install, but patience is a virtue. The main valve rarely causes trouble, and the air bags themselves have a failrly long life span. For anyone buying a 6.3, these are the two main areas in my opinion that you need to concentrate on, and budget for if there is no history of them being repaired, overhauled or replaced. If unable to do the work yourself, you should budget for at least $4000 on top of the purchase price, and is surely a negotiating tactic if the seller cannot provide evidence of recent work. You would also need to budget for fuel consumption. They do like to be driven hard, unlike a 6.9, which will accept moderate use. There is no comparison between the two. The 6.3 feels like it wants to run and run, whereas the 6.9 is a much smoother beast and doesn't feel like it has the same power. It doesn't, as while the HP is somewhat similar, the 6.3 has greater torque, and it is this that makes it seem to want to go hard.
There are other areas as Styria mentions, such as engine mounts, which are extremely expensive, and crticial because of the torque, so again, if these have not been done, they will need to be. When new, the torque put out caused the steering box mounts to be literally torn from the chassis, and a procedure was issued to install a reinforcing plate. Almost all would have been repaired during their warranty period, but it is essentail that the bolts are checked for tightness periodically. Other than that, the only other weak point on a 6.3 is the differential, and it is not recommended to wheel spin through left hand turns, or to put the car into reverse when the automatic choke is on i.e when the revs exceed idling speed. One tip here is to always reverse into your garage or parking space when you come home, so that you don't need to use reverse when the car is cold.