Is there any literally gold ratio between quality and quantity?!

[nikolgiorgos]

If you play carefully, you can build (and keep) a 5* squad of 11 players every season (or 9 5* and 2-3 4* god players from previous season).
Only with the sources of the game.
To do that (as you love the game and suppose you spend some time), read a lot many useful threads from the forum (from Tutorial and guides too).

Oh and is there any significant difference in improvement gain by training between 18 yr old and for ex. 27 yr old?

yes, there is a big difference between two players 18 y.o. too

[Avvy Jones]

I'm really involved in and probably addicted, too... Haven't missed a day without spending about 1-2 hours.

I don't really endorse spending money on any game, despite that I have quite limited budget. So bidding is rather problematic.
Thank you anyways.

Can you recommend some guides about methods of bidding and purchasing players ?