>Are there specific conditions to critically look for/at when considering a used D3s, obviosly including an accurate shutter count?
Some points I would suggest:
As James pointed out the D3s is rated at 300,000 clicks.. so don't get too hung up on that... while you do want a low count, even if it had say 150,000 clicks (which is a lot) it would take you about 20 years to shoot the equivalent of 20 thirty six frames rolls per month! (do the math.. if you don't think so)
The latest and great firmware (and the last) is 1.01... if it still has 1.00 no big deal... just means the previous owner didn't care.. didn't know how or was afraid to update. The 1.01 update included mostly issues regarding "movie" recording so again.. no big deal if it is 1.00
While the old Nikon FTn had "brassing" (which some consider a badge of honor)... the D3s unless it was really abused is not going to show brassing.. but watch out for the leather grip coming apart or peeling from the body... even if so no real problem unless you can see damage to the body metal underneath.
Metering Switch: This is a must to check. On some early production models of the D3, D3s and even the D3x, that metering switch internal contacts sometimes slipped... you set it for spot... but in the viewfinder it shows and meters for matrix or vise versa... move the switch, look in the viewfinder and see if it matches the metering mode you set it for.
The CF memory card door and release switch: Make sure the are no obvious signs of damage to the card holders, that the door pops open smoothly and that the release switch does indeed pop out the cf cards smoothly
Finally, no leakage in the battery compartment (also the clock battery) ... no damage to the threads of the tripod socket, make sure the flash contacts (both shoe mount and side socket) are clean and not marred, the 10 pin remote socket, usb socket and other connectors are not bent...
Also as James suggest if you go with "KEH" ... they have a very lenient return policy if you don't like the camera...
Anyway.. hope this helps.. if I can think of any other pointers i will post