I’ve got an old (C5-shape 2003-plate) Audi RS6 Estate (4.2 twin-turbo V8, not the newer V10 or current V8).
I would say it was perfect for MTB-ing, but I don’t want to get the inside dirty so the bike always goes on the roof. Could therefore get something smaller which was a little more nimble in the bends… but it is the perfect family car for us.
I’ve averaged 19mpg in the 6 years I’ve owned it (occasionally see 25mpg on the motorway if you’re lucky).
You can fit a tow-bar easily (as far as tow-bars are concerned it’s about the same underneath as a standard A6) it’s just not officially allowed a tow-bar. I recall discussions on the RS forum that this is due to the weight of towing a trailer, added with the power and torque of the V8 will just destroy the fragile gearbox if driven hard… box oil cooling isn’t sufficient).
Other owners have fitted one with removable tow-hitch for bike-rack duties though, but you’d have to discuss the legalities of this with your insurance company.
Buy something like this with eyes wide open (re: cost of consumables like tyres, brake discs and pads, servicing-espicially cam-belt changes and weak spots (turbo failure, gearbox failure… all possible and gearboxes are around the £4k – £5k region to get fixed by a decent independent) and then enjoy it).
I tend to like things like this as they don’t look too dissimilar to a standard Estate car (unless you spot the twin oval exhausts and slightly wider-flared arches).