I'm not sure it is really known how much effect the trainer level has. Training is more complicated in BB since there are the factor of minutes, each training type trains multiple skills, the training rate may be dependent on player height, as well as other skills, etc. Also it is generally best to rotate skills, so you won't find anyone with 8 seasons of training the same skill, or counting pop times.
I'd be concerned about a Level 1, based on my experience with a Level 1 doctor.
If you didn't hire the Level 7 trainer yesterday, you will notice something quirky about hiring staff in BB. The trainer for hire this week is a Level 1 Cody Parsons.
Another thing is that the staff is somewhat more like the assistants in HT - it is simply a weekly expense. So even though they have names and faces they are just a weekly expense. It is not like in HT where you spend several $100,000 every few seasons on a new coach, and who can't be transferred only fired.
So if you had hired the Level 7 and decide 3 weeks later that he is too expensive, you simply can switch to another trainer, and pay one week salary. There is a little bit of risk since you might have a Level 10 (who is even more expensive) or a Level 1 who is cheap but may be fairly useless.