As long as its been well looked after and is clean and dust free, not a lot.  The sensor should be dust free (no spots in same place on every photo).  Main thing to watch would be the shutter actuation count as depending on the camera it can vary from 20,000 to over 200,000 before failing.  There's a few places on the web that tell you how to check this and what each camera is capable of.
				
			 
			
		 
			
				
			
			
				MY15 Discovery 4 SE SDV6
Past: 97 D1 Tdi, 03 D2a Td5, 08 Kimberley Kamper, 08 Defender 110 TDCi, 99 Defender 110 300Tdi[/SIZE]
			
			
		 
	
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