Hmmm, a little decision you got to make there Markos...
You could decide on emotions or business, or a combination of both.
The deal seems to be a win/win situation, but the way I see it, the football club will be slightly ahead.
They have the space but no equipment or trainer.
They need equipment and a trainer.
You already have your own set up currently working just fine, maybe getting a little crowded at times.
Just had a quick look on realestate to see what you could get a place for and found this as an example:
Commercial Real Estate and Business for Sale
I'm know nothing of the location so may be missing something, but for $250/week for a 200sqm place, that sounds cheap.
Now obviously your time is worth alot more than $250 to train 20 guys multiple times a week, but you would likely get other opportunities arising from the exercise.
Then again, maybe it will not be as you initially wanted or expected it to be.
The problem is that when you make a deal, you have to stick with it.
With your own place, you can do whatever you want and train whoever you choose to.
Make your own rules and do the hours you want to, NO RESTRICTIONS.
If you had your own place (your name on lease or title), you will have zero restrictions and possible issues with anybody.
You can do what you want and attract the type of clients you want, footballers too.
This is the only potential problem I see going in with the club.
Arguments with management, disagreements, kid gets injured and blames your techniques, so the club tells you don't do this or that exercise anymore, too dangerous, etc.
Is extending your place definitely not an option, loan for $15k possible?