If I get a used big block 727 to learn to rebuild it myself, are there some years or original applications that are better for a performance trans?

Same question for valve bodies, because I know there is some change between 1971 and newer compared to earlier. Does it makes a difference if the valve body is going to get a mod kit in it either way? Does it matter if it started out as a 904 valve body if its still going to be modified?

What is a fair price for a rebuildable core 727? If I buy a used valve body separate from the trans, what should I expect to pay? The prices I see on ebay vary so much I don't know to think.