We've bought loads from Withams, the Ferret and Fox were before they did the sales, but have bought a Spartan, FV432 and 2 trailers, along with several stillages of stuff from them in the sales. You do have to be careful with anything you buy, it's sold as seen.
We like restoring vehicles, so expect to find things missing (engines!) or broken. Having had a vehicle restored by someone else before we will not go down that route again. Usually if it's come straight from the MoD it's not been bodged too much.
I have heard the stories of people buying vehicles without seeing them and realising something major such as the engine is missing. Everything can look good in a photo, my advice is always go and check it out!
Chris