The 123 came out as a 1967 cub cadet. It was the very first hydrostatic transmission on a cub cadet that IH put out, so you have a great find on your hands......a real piece of history. It was also featured in a traveling circus (see photos).
I usually just check for mice nest in the engine tins, check the oil and put some new fuel in/drain old out and then put a battery in to see if it fires up. As soon as it does (if it does), then I shut it down and then start replacing filter, possibly had gasket, rebuild carb, etc. once I know that the engine doesn't need rebuilt or she doesn't need major work.
Congratulations on your first real garden tractor purchase!! The 123 was one of the last all metal cubs (aside from the seat and steering wheel cap, that IH produced.
Cub Cadet 123