The key to the sankey to corny switch is the 1/4 MFL tailpieces that you attach to your existing sankey tap and the 1/4 flare swiel nuts that attach to your lines as shown here from Kegconnection.com.
You might also consider a
90 Degree Beer Thread Adapter if you are close to the top of your kegerator with your existing sankey tap. The flare tailpiece and swivel nut will add a couple of inches to the top of the sankey tap.
Unless you absolutely must have a shiny, undented and unscratched keg, any used corny that is holding pressure will be fine. You are really paying for looks if you go with a brand new keg. A used pressure tested corny will give you trouble free beer for quite some time, and poppets and o-rings are simple to replace if you start to have problems down the road.