1) a good goal keeper is important but so is a good striker as well as a good defense and midfielders. What u should do is have at least one 5 star player in each position.

2) when u buy, u can only see the quality of the player so look for the highest quality, but after that, when u create a formation, use the players with the highest form; some 5 stars might have less form that 3 star players.

Also if someone is expensive doesn't always mean that they are good.... I bought a 80k 5 star player and he has been MOM for 12 matches and has scored 17 goals. Look for players with 4 or 5 stars that r not that expensive. If u want more chances of winning, don't change the formation or player often, it makes them lose morale fast and gives the team higher chances of failure.

Finally scout list players can be good but usually they r pointless.