Hi potential 4matic owner.
I was recently in your position and took the plunge and bought a not working 4matic on the basis that being a Mercedes product it would be fixable and reliable. I may have been wrong. The 4matic is fixable but any forum search will show that there are many and recurring problems with early 4matic cars. Mercedes later changed the design significantly on later models and these appear relaible and more what we expect from Mercedes.
The early 4matic system is clever but a little complex and I have not found anyone anywhere that truly understands them and is experienced in fixing them (if anyone doubts this please send me the link to them!). While I believe I understand how mine has failed and I believe I can fix it, it is costly and will not increase the value of the car anyway.
I have converted the car to 2wheel drive and may revisit the 4matic if or when I can get some good advice.
Converting to 2WD is fairly easy, if anyone wants to know how, I am happy to give details....

Regards,

Farmbenz