One difference between the old and the new frames is that the new ones don't have a trigger over travel screw. For that reason, I would prefer an older frame.
As for whether to buy just a barrel or the entire gun with a frame; that is easy. Buy the entire gun. As your addiction grows you will accumulate more barrels and won't be wanting to switch barrels on frames all the time.
Also, if you buy Warne Weaver style bases and their QD rings, you can easily switch scopes between barrels.