+1 apollo7. I can imagine multiple strategies working for this, but I generally follow the formula that he suggests. Though I don't always achieve it, my goal is to get coaches who are 70+ in their primary skills in levels HiA thru AAA. For the AAA group, they should be high 70s or maybe even low 80s. Although I don't quite know how to measure the results, Patience and Discipline are also very important. I shoot for guys with 50+ ratings in both, especially valuing numbers above 80. Of course, I sometimes get beaten in the hiring process and end up settling for an inferior candidate...
As far as approach goes, the problem I run into in hiring coaches is whether super-high Patience and Discipline (say 80+ in both) make a hitting coach with a 63 hitting IQ worth taking, or whether a coach with an 81 hitting IQ but Patience and Discipline in the 20s would be better...I honestly don't know the answer. I've watched good GM's do both things.