The first time I played (with the experience watching my bro played before) I bought a 4* defender on money gained from cup, and then another 4* mid season when enough cash is made. And it was enough to win the league without problems.

The prize money at low level is very significant, it's actually quick buck. Try to buy younger 4* and sell them when they reach 3* two levels later, at low level you will actually earn more money from selling that player than the cost to buy and wage combined.

Until you are financially fine don't target at the 5* players yet, they train slower. The fast trainers are too expansive for their added power and too risky if you only have cash to afford one or two, you'd better get five 4* with that money instead.