Isn't the transmission a 722.9? If yes, they are extremely robust, and very tolerant of service abuse. There is no reason to fear transmission issues resulting from doing a fluid/filter change on a tranny that has been ignored. If it has never been done, it is likely that you will want to replace the engine mounts and transmission mounts. If you do it yourself, it can be done fairly inexpensively.
The same is true for all maintenance on a Mercedes. Pay a dealership $300 for an A service, or do it yourself for $50. Personally, I don't know what all these people are talking about, when they say it is expensive to own, and to run away from it. I bought my E350 4Matic with 46k miles on it. It now has >88k miles on it. After fixing a few inexpensive trim items, that the previous owner had neglected, I have driven the E350 for the cost of tires, filters, fluids and such. Never a single problem. My '07 SL550 took a little more than that. But nothing I couldn't fix myself. And nothing that cost me more than $100 here or $200 there. And it had over 100k miles on it.
I would jump all over it, if I could get it for the $3500 price range, and the PPI doesn't come back with any big issues. As you say, I don't see how you can go wrong. Drive it for a year or two and sell it.