i have problems with the value of those players, because i am buying most times "complete" players and don't start from the beiginning which i would recommend for you.
So first you should look on size, for centers size matters for there training speed a lot and you will probadly keep them a bit. I won't buy players who are smaller then 2m.
Centers are reaching there potential earlier then other players, so i would look for potential around starter+(which should be trained to 35k salary, and very solid players), if you find a good looking prospect with less potential you could buy him but keep aware of his training progress and when he starts to miss skillups sell him ;)
For the age i will look for 18/19 year old to start their training.
Startingskills, as i said i have no idea about the value so i can not say exact what you look for - but it gives 2 philosophies in buying then. You could look for players with good secondary skills, who are difficult to train(playing position + size) and less good primaries, these player will be weaker especially in the beginning but probadly more comfortabel in salary later on.
The opposite way is look for centers with good primary skills, and ignoring the secondary one, especially with the old engine(the new one i doesn't know good enough) those guys made a very good job - but you won't get that much money when selling them with the sideeffect that they are cheaper when you buy them.
Maybe you could combine both strategies a bit, especially the PF should have a bit OD and JS what you won't train that often.