Hi @houb ,
What parts have been affected would be unknown until it was opened and inspected as to what damage has occurred. This is the risk you take when buying water damaged devices.
It may be minimal or extensive. Not powering on does not give an indication of what else may be wrong. Before buying the camera, you might try asking the seller a few questions about the camera’s condition e.g. has it been opened and checked for damage by them or not, i.e. have they been trying to fix it themselves and gave up which can affect a lot of things if they weren’t careful, how long ago did the water damage occur, (the longer it has been the more possible corrosion damage there is) etc and base your decision on their responses and your gut instinct.
If you decide to risk it then once you have dis-assembled the camera you will have to clean all the affected parts using Isopropyl Alcohol 99%+ (available from electronics parts stores) to remove all traces of corrosion. Do not use "rubbing alcohol" as in some cases this is only 70% IPA or less, can contain additives scents and is not as effective. If you do have to use it, check the label to verify the amount. The higher the percentage of IPA the better.
Here is a link that describes the process. Electronics Water Damage
As always with electronics, especially surface mounted pcb be gentle when handling and especially when brushing away the corrosion. You do not want to remove any components from the boards. Also if it has been a while most probably the battery will have to be replaced as even if it hadn’t been affected by the water it may have become depleted beyond recovery.
Once you are satisfied that all the corrosion has been removed and the camera re-assembled, then you can proceed with finding out what works and what doesn’t. It may be unwise to just fix the most obvious problems and then apply power to the camera to test it without making sure that there are no other problems lurking elsewhere first
Here is a link to the ifixit Canon EOS 5D Mark III Repair guides which may be of some help. There is also a parts catalogue (scroll down to Documents section in the link), which may come in handy.
I’m not trying to put you off at all, but you’re the one that’s taking the risk. I hope that it is just a simple fix.