JEFF - Welcome to the forum. I can answer a couple of Your questions. The only ONAN engine IH used was the 19.9 HP in the 982's & very early 2072's. The engines actually stay cooler & cleaner with the side panels installed. IH mostly used Kohler power, K-series cast iron single cyl. and the KT-17, both series I & II in the 682 & 782. The 482 & 582 had a 16 HP Briggs horizontal opposed twin which was actually a pretty decent engine. The 1872 & 2072 used the newer Kohler 18 & 20 HP opposed twin cyl. engine, I'm not positive but I think they may have been MAGNUM 18 & 20's.
The decks known a 44, 44A and 44C, 50, 50A, & 50C are all basically the same, the "44" & "50" are generic descriptions, the "A" decks fit the older tractors that use a 3/8" wide drive belt, 149, 169, 1450, 1650. The "C" decks have the driven pulley on the deck designed for the larger 5/8" wide drive belt. Plus there are some changes to the deck itself to allow the deck to fit the mule drive bracket on the tractor. The 44 & 50 deck skins are "Drawn" from thinner steel than earlier IH decks so they also tend to crack more frequently is stressed areas, mostly where the rear gauge wheel brackets bolt on, and sometimes around where the rear mower deck hangers bolt on. The spindle bearings are greasable but the OEM bearings are a unit bearing, also known as "Waterpump" bearings and are expensive to replace. But cheaper alternative replacement bearings are available. With a little work they do a fine job of mowing.
I mow just a bit over 2 acres, and have about 60-70 trees & bushes, LOTS of trimming, takes Me about 2-1/2 to 3 hours to mow with a 982 & 50C deck and I don't hurry. Also My 50C deck doesn't "FIT" real well some places in My yard, My old 129 & 44A deck actually mowed the same yard better, on less gas, and took maybe 10-15 minutes longer.
If You want a rear tiller I'd for sure get a tractor with hyd. lift, Tillers are heavy, I'm not a big fan of the "Quiet-Line tractors", so I'd recommend a 149 or 169, 782 would be nice. The SGT's, 982, 1872, & 2072 are also smooth, have hyd. lift, and most have aux. remote hyd. outlets, and all but the 982 have power steering. But they're kinda hard to find in decent shape. I looked off & on for 3-4 yrs before I bought My 982 back in fall of 2000.
The SGT's also have a Cat O 3-point hitch available, rear 2000 rpm independent PTO, and individual steering brakes, a 48" rear 3-point mounted tiller was available for them but they're pretty rare. To mount an IH tiller on the other tractors gets a little complicated, special brackets, gearboxes, lifts etc are required. Dad had a tiller on a dedicated 129 that I ran a little, the 129 ran it just fine, but I'll let someone more familiar with them describe them.
Finding new & used parts to keep these old Cub Cadets running is REAL easy, Plus they don;t take much to keep running. Attachments are also easy to find. And attachments for the vintage tractors Your looking at are almost all interchangable. You may have to swap a pulley if You get a 782 but that's about it. IH CC's IMHO have the easiest attachment mounting system of ANY garden tractor also.
