You are right- the 1811 looks to be basically the old International 782 with an 18 HP Kohler Magnum twin in place of the 782's KT17 Kohler twin. That and the 1211 are just the same tractor with a different engine. 680, 682, 782, 1282, 1211, 1212, 1811... all just about the same tractor with either a different engine bolted in or hydraulic lift accessory. They all use the old style mule drive and "C" series mower decks. MTD jumbled the model mumbers around a lot which makes it confusing, but more or less in the late 80's MTD switched the tractors and decks from the International designs to designs of their own that are much different and have little if any parts interchangability with the International designs.
I don't know if it's just the history lover in me, but it sure seems like the old International Cub Cadet garden tractor designs were more rugged than most of the stuff MTD has been designing. Melting plastic hoods and panels, exhaust popping, lifter noise, or slipping or weak hydro transaxles were things one just didn't run across when they invested in a new International Harvester built Cub Cadet 25-30 years ago. I'm not just faulting MTD, all the other makes have cheaped out what a true garden tractor should be also.
For one who doesn't mind a bit of mechanical maintenance or light repairs here and there, an old classic Cub Cadet is a wise investment.
-Fordlords-