Any Advise for Power Traiining

[Spyros]

What kind of investment is needed for power training the substitute players (rest main squad - train the rest) for a star up?
Are 50 rest packs enough? do i have to train them each hour all day ? can they take all 3 kinds of training without braking a leg or other injury ?

If I need more than 50 packs does it worth it or is it better to sell and get scouts?

any pro tips will be godsent - players average age 20 & average quality 47 and I need to get em to 50 - thnx in advance