I always look for 4 stars or above. At least then you know if they dont maximise there potential their still going to be fairly decent and playable.
If your looking to buy, play, sell then 3.0 -3.5 id recommend. That sort of range for an English player when they hit there top potential could bring you between £5m-£12m.
I love picking up defender newgens for upto like £2.5m, U18s till their too old. Loan out for 2 seasons now the players are 20 you can pick from your crop.