Venting re" Full Frame

Pentax 35mm f2.8 macro LTD?
Pentax 35mm f2 or the newer f2.4
samyang 35mm f1.4
Sigma art 35mm f1.4 (getting great reviews) 
All would give you equivalent to 50mm FF. 
Or how about some of the 24-60 or similar type zooms?
 I haven't used the 60-250f4 but the 50-135 is a stellar lens and I don't see why the lack of a tripod collar would stop you using it with a ring flash when you can get L bracket's for the camera body.
The sigma 50mm 1.4 is apparently more like a 43mm or so.. that could be good?
I had a FF camera for a while and the only difference I noticed really was more DoF.. the perspective of the image is controlled by distance and using an equivalent lens gives you the same image.
As for image quality, I found my k5 to be better than the 12mp FF nikon sensor, and while it loses out in resolution to the 5dmark2 it has much better dynamic range.