I believe that when they reach 13,500 miles the cars will be sold as they have a sticker on the dash advising that Avis must be contacted at this mileage. As most will be used for fleet use then I would think some will have got near this mileage already that were supplied in December. Be warned though that from some accounts on the internet, some have abused the hire cars with reports of smoking the clutch in one case because they hated the car, to quote.
However, the Avis fleet is primarily S and SE spec cars, there are no TSE spec cars, which means you won't get the reverse parking camera, blue-tooth hands-free or leather seats, but you could retro-fit the 18" rims/tyres from a ZT as the tyres are the same size. There are some good deals ex-demo and pre-reg on the TSE cars, although many dealers used the SE as demo, my local dealer has a white GT SE with around 6k miles on it at £12,900, and I know the person who put ost of the miles on while their TF was in the bodyshop.
The plus side to the Avis cars is that they will be run in and you will be getting good fuel economy as this takes time to improve, but on the downside the current new cars are in a lower tax band than the ones manufactured up till April 2012.