I am no expert but here is what I have learned over the past 4 months!

When to buy is tricky, but a good bet(if you have the time) is to be up for Server reset(2:30am) and then see what players are available, this leads into point 2 gem or dud.

Individual results may vary but I find these players are my best performers, with constant 7-9 ratings in games even OOP. From the market itself you cant tell, but like RL any player can be a flop(just ask Fernando Torres!)

My idea of power training is using both green and red packs to gain skill points as fast as possible by training him everytime a training routine comes up. A good way to train is to arrange a friendly to gain packs, skill points and rests if you don't get a yellow or red card!

My best Nord gen has come from the academy and averages 0.95 goals a game
Hope my noobish ways are of any help to you!