You already have an S - Class 126 and if you are looking for a stable driver, I would go with the newer model, the 300E, only since you like it. Me personally, I like the S-Class only for me. The 420 however, was maybe the overall best all around 4 door sedan ever made, just my 2 cents. A car 7 years newer has the odds in its favor to have less problems for you, but you have to do your own diligence.
As far as a loan, some credit card deals are offering 2.9% for the life of the loan and never goes up, but look around and do a search on the internet. What about your credit union where you work?
'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)