I see every day topics like this and i think that i can give you answer (not sure just from my 1 year exp).I am 13 lvl and i play with 1 sistem all 13 seasons.When i need player i always try to buy most expensive,i prefer to have 1 top player than 5 ****s.You know CR7 is AMC/AML/AMR and his price is big but in footbal team have so mutch AMC but not many cost like him.You can have full 5* team but if heyr price is lower for your lvl they will loose from 3* better quality teams.With this strategy i have 7 CL winner and 3 time runner up for 13 seasons,win 11 leage and just 1 second place.For exampe now my team cost almost 900 mln 81 quality today i win 4:0 vs team who cost 483 mln 83 quality i play 4-4-2 he 4-1-2-1-2.I am not 100% sure about this but for me work good.
If you can be in top 4 better stay in same lvl next season..