My impression (from very limited experience) is that autos are only good for 150k miles & then then you're into bills of £2k to replace
as long as you can drain/replace the transmission fliud and filters regularly then an auto box should last indefinitely,
unless you somehow put it in reverse whilst travelling along the m40
on older mercs the atf and filters were a 40k service item, newer ones are sealed for life, hence and old merc will still drive long after the body has rusted off, where as a new one will (probably) get binned through box failure before it reaches 100k. same story with fords. mk2 granada box will do 2-300k and a couple of seasons banger racing, a mk3 box will be dead by 100k. had 4 mk3 granadas previously, and not one had its original box in :-/
if your buying the car for effortless driving get an auto, if you want to drive it,get a manual.