I would personally focus on Defence as there are less skills to train. Also, this would cover your Offensive Lineman.

Another choice is Fitness. A team who has players with ridiculous Stamina, let alone Strength and speed, will do very well.

In regards to numbers, it's up to you. If you can get a level 20 coach, do it. I would rather go all out for one, than just a little with two.

Guru.

When he's hired, it will take out the signing on fee.

The 10 weeks is just an incentive. It makes buying coaches harder

Hope this helps.