The higher their value the faster they train pretty much. Getting these players and training them up also allows you to train their skills based on position. IE not having equal defensive skills for ST's or OFF for CBs. They all start out fairly equal but as you train up your Nordgen the gap can grow so that even if you have a 7* ST his off/phy-mental skills are in the 8-9* range making them even better.
It is far more satisfying to have a core of players stick around a few seasons and set records.
Just like with all signings some of them will flop but you can always get your money back and try again when it happens. One great thing is that at the lower levels nobody really bids for them (their price scares people away) so you get them for 1T plus cost almost always. When you put that 70T you would have spent on a Scout into training them they will be equal in skill but over the season they will grow much faster and continue to grow with you as you lvl up unlike the scout.
Farm as many green packs as you can along with tokens for training.