I wouldn't worry about one spot at f/16 after 400 photos. You can also ask the seller if the camera had the Nikon D600 recall/warranty repair. If not you can always do that. If it did and you find a problem, you can tell them again and hopefully they will do something about it. You can tell them about the viewfinder line too if you do.
I also wear glasses and can't really help, except adding that I've tried Nikon's viewfinder magnifier and it was worse, so I wouldn't get that.
|Red umbrella by Jose Olivares|
from Black + White +1
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from annual bad Santa photo challenge
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from -The Frozen Lake- ( In Full Colours Only)