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.
|New Forest pony by Dutch Newchurch|
from Equines in 2018
|Leader of the pack by Wu Jiaqiu|
from My Best Photo of the Week
|Czech Crown by Tobik|
from Coins - Macro only