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.
|2014_1211_140657AA by old shutter bugger|
from The Bride
|Overloaded by NZ Scott|
from Your City - Delivery Boy
|Barley by Will B Milner|
|APPLE & ROACH by TX Photo Doc|
from Delicious - Unpalatable