The primaries are where its at to pretty much advance quickest out of a low level league no? I mean, if you are in a V, IV and even ,any III's.. why spend a season pumping up your power foward's passing and OD incase he matches up with a good shooting pf for jumpshots or to pass the ball better or center's handling to produce .4 less turnovers per game, etc?
depends what you plan with your C. Just for succes is would be really better to train him in Rebounds etc. then Handling passing etc., but if u have a player with an ambitious Potential with you u want stick for a longer period of time i would train this things first because in bottom div there are often big differences betwen older and younger team so u got pretty easy matches and matches you could give up before the tipp off. Another point for training it first, is that an Rebounding/inside Shot 14 Center is much worse as PG then he is on his main Position, in the beginning of his career the difference of his performance out of position could be pretty low.
So thats about training, for buying players i would say get as much secondaries you could get but don't pay for it.