When should I invest into them?

projectrracing wrote: after you have read the sticky thread, 'spilling the beans 2'.

Touché!!!

Read the StB version 2 and when you have done everything mentioned before 'Hire trainers' then hire trainers.

And since you are asking, then hire only fitness trainers (after you have built the fitness center) and hire a level 20 trainer. You must have at least 390.000 in you bank account to hire a level 20 trainer and you should not buy less. It's wasting good money incrementally upgrading your trainers.