- I don't think there are any plans to raise the training levels and there is no reason to do that.
It's better to wait for a new feature, starting with a new training style and a different profile/ development of players.
I wish this time individual skills- character of a player, to have some important role in gameplay.
But this needs a lot of work so I guess maybe is some future plan like TE2019.

- do individual player attributes make much of a difference at all?.Or is the quality much more important ?
Quality is more important but it's not enough.
First of all, I consider "higher" quality from 20% (1*) and above.
But also matter other things, minor or major like coaching and luck.

- Is it possible to make the finances of the team self sustaining without having to pay or sell the odd player now and then?
if you have finished the stadium, playing in CH.L., some semi/finals every season, organize a friendly here and there, adjust the tickets
then your finance will be fine.

- Is there a level where people find it starts to get very difficult to play the game without having to start buying funds to go into the squad?
It depends.
If you 're an active player and log in every day, if you can farm some greens, if you are in a good asso group (Gold)
then you can gather 100-120 tokens and 600-900 packs for free and have a competitive team.