Your two main criteria -- (1) 7 seats, (2) $15k -- seem to eliminate the ML as an option.
While it is true that the ML was available with 7 seats, they really are not very functional. Access to the back is very cumbersome (although improved in 2002) and leg room is almost non-existant. Also, for $15k, you will be limited to early model years that were most problematic. And, if you are limited by a budget you probably won't like the average $1,000 per year maintanence costs (normal maintanence + typical repairs over 5 years of ownership).
If you still love the ML despite these drawbacks, then try and get as new as you can afford. They improved each year. Reliability was consistently low until about 2003.
1. Pls can some one advise what is the best engine?
ML 500
2. What are the main problems or weak points?
Search. There are about 30 common problems that are found on the 98-01 MLs.
3. Are there any good websites on the M class?
Here, BenzWorld,
www.whnet.com/4x4
4. When did the facelift occur after the initial version in 98.
Improvements each year. 2002 was the major update.