Ironically Big Red's gearbox just blew up about 5 minutes ago
Went to reverse on to the driveway, nothing, nothing *bang*. Bugger
S8 in bits, Allroad also broken, obvs.
That grey one on Gumtree / ebay is a basket case. It started off as a shouty 'future classic £15K' about a year ago - there's a reason it hasn't sold. Assume it needs £3-4K for a gearbox refurb, and there is plenty else wrong with it if you look closely at the pictures. I'd consider that maybe worth £2-3K, subject to a very close inspection.
Never assume a gearbox is just the F125 until you've verified fault codes. There are so many things which can cause limp mode on these from the F125 having dirty contacts, to the TCU being wet, to shiny chunks in the sump. There's not really a mileage trigger - my first D2 made it to 200K before the gearbox went bang. The gearbox that just went pop in Big Red has about 95K on it (160 on the car). It depends a lot on whether its been serviced or not, whether the car has been regularly used (F125 and torque converters don't like being laid up) and also there's a considerable element of luck in how long the internal stuff lasts. Oil and filter doesn't prevent external failures, but it does prolong the life of the internals if done properly, which is a whole 'nother conversation. If there's no history of maintenance, or a gearbox refurb *by a reputable ZF agent* then keep £4K aside for when it does fail.