Check out all of the available sites on the internet; GunsAmerica, etc. If you find one for $5,500 in excellent shape, buy it. $6,500 appears to be the minimum.
As to the guns themselves, a 16 gauge on a 20 gauge frame is going to kick you more than a 12 gauge will. Only way to avoid that is to use 2.5 inch light loads, in which case, you should simply get a 20 gauge and save some money.
Before the 16 ga. fans jump on me, I have a 16 ga. Fox that I really like. smile