Shawn L., I reckon IH bounced back and forth between the internal brakes and the disc brakes several times while they were manufacturing Cub Cadets. I think some of the earlier ones had the internal brakes; my oldest one is a 106, which has external brakes, and pretty much all the wideframes (1x8/1x9 series) that I've seen have had external brakes. They got back into the internal brakes during the Quietline series, or maybe very late in the 1x8/1x9s, I'm not sure when, but I also think it's possible to find Quietlines with both styles of brakes, similar to what you say you're seeing on 782s.
As to which is better, I don't know that I can answer that for sure. I used to think the external brakes were probably better, but now that my 1000 is running (my first Quietline, and first tractor with internal brakes), it seems to stop smoother and more positively than any of my disc-brake tractors, so I'm kinda rethinking my position on 'em just now. The internals are easier to adjust, and the internal discs (pucks) are way cheaper than new pads for the externals, but you do have to open up the rear end to replace the internal brakes and you don't have to do that to fix the externals.
And on your seat recovering question, I don't know of anybody that does it. I did save the original seat from my 106 in case I ever ran across anybody that could do it, but it would have to be considerably cheaper than buying a new seat or an EXACT replica of the factory seat to get me to go that route now, as I've been pretty happy with the CC replacement seats I've bought.