Actually OK is not an abbreviation of Okay. OK was the way the word was spelled originally, before Okay/okay/O.K. ever existed. Ok with a lowercase 'k' is not correct, however, even as an abbreviation--it's simply nothing. But OK and okay are used interchangeably even in somewhat more scholarly works--NY Times, etc.
We are agreed on numerals vs. numbers spelled out. IIRC, I think we were taught in school that 1-12 you spell out, anything higher you can use the numeral? I tend to spell out anything less than a 3 digit number, though.

Silent Warrior, do you have any examples yet of your progress?