Transfers / Values / Most important players

So i am building a young side and have some questions.

At level 2 will my young players actually lose transfer value or just a star?
I have many players within a point of another star and will keep developing from 19-26 etc and wonder if developing them will gain me money when i sell them on at say 21.

Which positions do you prioritise? I need a good 2nd striker, MR and two defence. The keeper is likewise only 4 stars, should i concentrate on the keeper and striker or how do you tend to play it?

Do i buy now with 40% of a season left when 600k can get me a 24 (near 6 star player for this season) or wait till the start of next season where i get a full season to develop the young player? Also, how much might that cost? I can get 3 good ones for 1.4mn now and might only get a single player for that next year.

Is buying a player with a 4 or 9 and close to a new higher star rating worth doing? Buy them, get 15 skills and then make a profit?