It depends on the price you're willing to pay. If you're just starting out and don't want to spend too much right away, you can buy a 18 year old with star/allstar potential and main skills between 5-7. You should be able to buy these for a couple 1000 bucks. It's not that bad if they have 1 skill that is below mediocre, but just check what is available at what price, determine the budget you want to spend and try to find the player(s) that approach what you aim for best.
For older players that you still want to train it gets a bit harder to say. I don't have much experience in buying and training older players, but as a very general guide line I think you can count the total skill points of a player (without stamina and free throw) and take 50 as a base for 18 year olds, and 10 extra for every year they're older than 18. There would still be a lot of exceptions to this, but that would be too long to write down for now.