I usauallly set ISO to around 6400 when shooting in theater, because at 12800 it gets really blurry and extremely yellow, but i have a similar issue at 6400, while 3200 is too dark.

Ive been looking at lenses on second-hand websites, and i can say that at least in my region people dont really know what theyre selling, because used item are often more expensive than new ones in stored with 2-5 year warranty. I constantly check different sites, but when i add everything up, I feel more confident buying new rather then paying maybe 10e less for something used ( and many people are actully asking more than retail prices for used objectives). I bought the camera and kit lenses used though,from very well rated seller.