Lee,
Starting in 1996, the W140's went to a digital powertrain management system. There's good news and bad news.
Good news: Your car has an OBD-II connector under the dash, near the steering column. This was mandated by US Federal DOT law and will provide you access to basic code data. Many simple scanners can read these DTCs and it will give you "pointers" to things that are wrong. You need to keep in mind that data obtained via the OBD-II system generally won't provide "the answer" if you're having a problem, but they will point you in the general direction. Just about any OBD-II scanner will work on this. I use an interface and software that runs on my laptop so I can see the data more easily and monitor some functions (like the O2 sensors) real time. Again, this system will give you basic code information. You'll need some experience to translate this information into knowledge. Run a search on OBD-II and I'm sure you'll find much on this.
The bad news: Mercedes kept all the "good stuff" for powertrain management in a manner more difficult and expensive to get to. There is a 38 pin connector under the hood on the passenger side. I won't go into this because you said "inexpensive", but there are scanners that can read this data stream (including engine, transmission, climate control, air bags, etc) but they aren't cheap or simple. I have a Snap-On MT2500 that works well for me.
Hope this helps.

__________________
KenP

1996 S500 W140 Coupe