Anyway, my point was, people were throwing too much information at you, and you've just started to play so you can't know where it's all coming from. Why not just enjoy the game, and you'll figure out things over time.
Example - arena - Yes, arena is a safe long term investment and blah blah.. On the other hand, you need to invest money, and it takes a full season just to get that money back. And that's in your situation, with max prices. In lower divisions, it could take about three seasons just to get the money back.
Example 2 - salary efficient players - Sure a player need to have secondary skills to be financially feasible. On the other hand, it's not like veteran donkeys are useless, they do play well. And a salary efficient player cost millions on the transfer list, while a salary inefficient and old players cost close to nothing. You can buy them for a bag of peanuts, their owners just want to get rid of their massive salaries.
So my advice is:
1) Make the best team possible, compete(which is the point of the game anyway)
2) Forget about the other things
3) The other things will come to you by themselves