I do not know when the MoD is planning on retiring the rest of the fleet, but around 10 years ago they did a life extension program where around 1000 of them were fitted with diesel engines. Sabres were not converted and so were disposed of in 2004, along with a few hundred Spartans, Sultans etc which were not converted. The majority of these went for scrap as the market for these vehicles is limited and the MoD wanted the money. The same happened to the Strikers, where other than the handful of petrol vehicles which trickled out over the years, the entire diesel fleet was scrapped or used for hard targets.
Just recently they have scrapped a load more, deciding it was easier to just have the money there and then than have the hassle of selling them complete. I was told that any future vehicles which might go for diposal will be bought back by BAE Systems so no idea if any will come onto the market in future.
As for spares, you'll need the obvious, belts, filters, pads, wheels, sprockets, track, engines (at least a couple), gearbox or 2.
Chris