$600 is a good price for a good lens. I have had bad luck from Ebay. I would recommend purchasing only from KEH, B&H and Canon only. Ebay is a crap shoot...
I'm not sure what will help f4 and low light...
Your trying to make a silk purse from a sows ear...
Just enough Bugs to make it stink, buy the new one, what do you think??
Watch out!!! anything for free, has a bad smell to me.
They sold me a used camera for new...don't go there...
The next time steady your camera on a fixed object. You might be moving involentarily when you hit the button...
I have a Oben ct 2341 with the BE-117 Ball head. It is in my openion about a 4.5 on a scale of 1-10.
Don't get anything you can't afford to get stolen!
get a 5dlll and quit whining
set it back to factory default settings...
Like a fishing friend said to me once"your a long time dead"
get what you want and enjoy it.
Trade in your old stuff and get a 5d3 and some good "L" lenses...