If you have the money you shouldn't stop upgrading. The earlier you finish your upgrades the more resources you will have at your disposal - Money, Rests, Treatments, Youth Academy Players.

If you have a slot free and he is free of cost, you should add him immediately. He will easily be worth more than 1.5M next season.

If anyone better pops up during the season you always have the option to release him. The next crop of players that pop up during the mid season are not free and cost Tokens.