Welcome to the BOLT! I believe that there should be a number on the drivers pillar that states that it is style number 3124. (INCORRECT) 3124 is the designation of the Cameo, although their number on the side FENDER emblem stated 3100 for 55/56. By the looks of the fiberglass panel and the absence of trim holes, I think you have a 55/56. The hard pieces to find are the tail lights, rear bumper/panel and tire carrier. The pot metal is also hard to find not pitted or not cracked. Some reproduction pieces are available, but high priced. The key to whether you paid too much is what parts are missing and your intentions. From the pictures you showed and without much detail on rust and condition, it looks like a fair price to me.
Here a a couple of sites to view, also look in our gallery for other cameos and suburban carriers. Good luck!
1955 GMC SUBURBAN CARRIER SITE CAMEO PICTURES PRODUCTION NUMBERS MORE INFO
