Since you're still in H, I'd advise you to be patient - don't make too many transfers, don't sell any of your players until they are fully trained.
Download SPMT, use it and learn about your squad from it.
For staff I'd say you only need a good coach (cca 80 percent should be fine) - your players will train faster.
Until you start to make transfers, you don't really need a scout, a very cheap one will do (30 percent).
You need a decent doctor, physio & groundsman (60 percent), maybe physio should be better (injury prevention).
You don't need an assistant manager at all (imho, still an open question here on the forum).
Youth team coach is a very good investment, I think it is worth signing a good one and train him to 100 percent.
A youth center is a very good investment, in fact my advice would be to start building level 2 (a "professional YCC") right after you finish the basic one, and continue with the Hyper-modern (top level) center right after that (that means 500k for each one I think, so save money).
Youth players are the most valuable thing in the game and the more you get, the better.
Until you have like 10.000 fans, you don't really need to expand the stadium at all, so don't worry about that.
If some of your players' contracts are about to run out, I mean they have less than say 1 season (63 days) left, then perhaps you should give them a 3 year contract now while they are untrained - they will accept a low wage. Use SPMT to calculate how much do you need to pay them.
That's all I can think of now