For the $7500, you will receive a return of $8500 paid over 6 months (monthly @ $1416.00 per month), as well as 20% of all Bitcoin mined, paid out daily or weekly for the life of the operation.
So regardless of how the business venture goes the person who provides the loan will receive $8500 back over the next six months?
Any default, a local lawyer would be happy to help you.
"You can't sqeeze blood from a stone".
If you default, it would cost more than $7500 to even get you to pay back 1/10th of the debt you owe, and that's if the investor is lucky. No lawyer will touch a bitcoin default and trying to get the money back would be impossible. You have a car as your primary asset, and no liquid assets. In other words, if you default on your obligation, no force in the universe will make you pay it back.
This will be a business partnership. Looking for a genuine investor here, knowledgeable in Bitcoin. It's a contract either way, and with a lien on an asset, any lawyer would do it. Would take them an application for repossession in any Magistrates Court, a few hours work for them.