Dan,
If the 420 has been maintained properly I'd expect to see similar maintenance as your current 300E. The V8's are a bit tougher to work on due to space contraints and repairs usually take a little more time but still manageable.
The wiring harness can be an issue on the V8's, I just noticed deteriorated insulation on my wiring harness last week. It may not need immediate attention, but when it does act up expect to spend $500 for the harness and if you have a dealership do it expect to pay for 3 hours of labor.
Seeing as the car has low miles I'd pay close attention to the AC, if it wasn't used regularly it may have dried out seals.
Good luck should you decide to purchase....oh and I think you'll be very happy with the additional performance!
