That all depends on how you pack... Put your chutes on the side of the trailer, your boxes in the back of the pickup, the rest of your tack in the pickup and the nose of the trailer, and you can get by with a 16' - but it'd be tight. If you want to set your chutes and boxes and stall dividers and all on the floor of the trailer, you prob'ly really want a 24' with
2 center gates - use 8' for tack and 16' for cattle. The trailer we pull to shows is a 6'8" x 20' Circle D, and I can haul everything you mentioned and then some comfortably (without hanging anything off the sides), you've just got to plan out where everything fits. I'm very seriously contemplating trading up to a 24' though - the extra space would sure be nice.
GM only offered one diesel engine in the 2500 - 3500 series in those years (or any year for that matter). The 08's may have a different size engine, as all of the big 3 had to completely retool their diesels to meet emissions standards.