Up until level 4, lower rated players are quite adequate. I got 4 successive promotions with squads that only contained a couple of 4's. Most were 3 or even 2.

As regards the nationalities, my team was like a United Nations. I had players from Montenegro, North Korea, Bermuda, Uganda etc, and had some really impressive results. Three successive CL finals (2 won and 1 lost).

If you are doing well, don't waste tokens on higher rated players. Try to save for the higher levels. (level 5 onwards).