Hi Jago,
I have towed with a Golf TDI130 estate for five years now, with no problems. Sixth gear (2000rpm at 70mph) is not often sensible, but fifth is ideal for me most of the time.
I would say a service record is very important when buying, and try to persuade the seller to replace the cambelt etc., unless this has recently been done. This is a costly service if performed by a VW main agent.
Towing I average around 32mpg, but obviously this varies according to conditions; solo I usually get around 54 mpg measuring between fill ups, but this can vary between 50mpg and 60mpg on individual trips. I find the trip computer is about 2mpg on the optimistic side.
I also tow a boat, which involves slipway hill starts when recovering it; even with this, I have not suffered from the problems with the Dual Mass Flywheel which have been reported by others, and the vehicle has now covered 71,000 miles (I am touching wood and have my fingers crossed!)
Problems to date have been one failed door lock (LH rear, fixed under warranty,) a broken dust cover on the rear hatch catch mechanism (fixed under warranty and a design fault I suspect,) and it now has a temperature sender problem which needs fixing, but does not affect things greatly it seems. Not too bad for a vehicle coming up to 6 years old (end of May) and with 71,000 miles under its belt.
Oh, and performance towing is excellent, with maximum torque at 1900rpm; it is capable of surprising some so called 'better' tow cars for both acceleration and uphill.
Good luck if you decide to go ahead and buy it.
Enjoy your holidays,
Trevor M