The numbers you are refering to are the model numbers of the various tractors, and for the IH built cub Cadets identify the horse power and the type of transmission.
The first model has no number and is commonly called an Original, and is a 7 hp. The next model is the 70 and 100. The first 1 or 2 digits identify the hp, and the last digit the type transmission - even number = gear drive, odd number= hydrostatic, with a few exceptions. Model 70, 71, 72, 73 are all 7 hp, but are all gear drive. Basically, the higher the last number, the later the model or series of tractor. Example, a 122 is a 12 hp gear drive and a 123 is a 12 hp hydro of the same basic model and series (the second series of numbered tractors) tractor which also included a 102, 10 hp model. The 122 and 123 were replaced by the 124 and 125 of the third series, which also included a 104 & 105, and the numbering sequence keeps going up with the later series until you get to the 1000, 1200, 1250 etc. They are a 10 hp gear drive, a 12 hp gear drive, and a 12hp hydro, in that order, and since the number 100 had already been used, and the number 125 had been used, another 0 was added as a fourth digit, and the third digit used to identify the transmission type. Next came the red 82 series, and nobody including IH understands the numbering system for them.
Now that all of that is as clear as mud, you might want to read through the FAQ's here.
http://ihregistry.com/odfaq/index.php?p ... t&cat=5#a6 and it may help answer some of your questions.
And to answer your question about what to look for, sounds like a 149 with a 44" or 50" deck is what you are looking for, but you only want a deck that big if your ground is pretty smooth. The larger decks are good at scalping if there are bumps or dips or uneven ground to mow.