The two major differences between the '97 and '00 are the engines, and the vehicle electronics. The M104 in-line six is arguably one of the best gasoline engines produced by MB. It's got great torque and is very DIY friendly. I wouldn't let the head gasket issue scare you away from the '97, as long as the car is in good shape. The M112 V-6 is a good engine as well, but like the M104, has it's own inherent issues. The most serious, IMHO, is the possibility that it may require a complete tear-down and re-ring at higher mileage, due to oil consumption. I've seen plenty of well maintained M112's come in with stuck/worn piston rings. MB will usually help out with the cost of the repair, as long as there is proof of maintenance and the mileage is less than 100K. After that, it can get pretty expensive.

The differences between the electrical systems are like night and day. The '97 is a pretty simple, non-networked (almost) system. Anyone who can read a wiring diagram and use a multi-meter should be able to diagnose most problems that occur. The '00 however, is another story. Just about every accessory (seats, windows, sunroof, instrumentation, central locking, interior lighting, etc....) is controlled via networking. To put it simply, there is no direct electrical connection between the switch and the consumer. This can make diagnosing electrical faults nearly impossible for even the bravest DIY'er.

Lastly, both the '97 and the '00 are equipped with the 722.6 five speed transmission.