There will be many slightly different opinions on these type of issues. There is really no correct answer. The common wisdom is level 4 or 5, which you have already read about. Level 5 is however pretty expensive.
If you already have good trainees, you can certainly consider getting a level 4 trainer. If you are still in the process of finding trainees, you could start with a cheap level 2 or 3 guy and replace as soon as you get your training started. Remember that you can train free throws and stamina (and game shape, but generally don't) as effectively with a level 1 guy.