Hi Blackjack21,
As someone who was once in a similar predicament, the 1999 M-Class can seem to be a good first car for your son. The original M-Class debuted with many innovative safety systems. Stability control debut in the original (1998-2005) M-Class for the first time in a luxury SUV. Side and front impact airbags also ensured that the 1998-2005 M-Class had a 4-star EuroNCAP safety rating, which was outstanding at the time of launch, and still very good compared to current models on sale.
Reliability ratings are mixed for different parts of the car, and as a result you'll find some conflicting info, however the 1998-2005 M-Class was received with generally poor reliability ratings. Some of the engines have great reliability whilst others have average. The most common and entry-level model in the US in 1999 is the ML 320, powered by a 3.2L petrol V6. This engine was reasonably reliable and was seen across the Mercedes-Benz model range, including the top-of-the-range flagship Mercedes model - the S-Class sedan. An attached video review by Top Gear UK's Jeremy Clarkson shows that he found an alarmingly large gap between the boot lid and the bumper.
I have travelled in a 2002 (facelifted model) M-Class and even after a supposed major refresh to improve reliability and quality, it still has had several issues from the fuel pump display to a fuel leak. It rides reasonably harsh and is a very loud car, even from the inside.
So as you can see, the 1999 M-Class wasn't a tremendously reliable car. You'd be better off looking at the 'W203' style 2000-2007 C-Class sedan. It featured even better safety ratings (EuroNCAP 5 star) and was generally more reliable. It will be more efficient and is more of a driver's car. I think as a first car a sedan is a lot easier to handle than a large SUV. I've travelled in a 2004 C-Class and (as I mentioned before) a 2002 M-Class and I'd take the C-Class in a heartbeat. It is cheaper, more fuel efficient, safe, more comfortable and represents very good value for money.
Hope I helped you :D
John