If I was your uncle, the car title would be in my name only. If the loan is unsecured, it has nothing to do with the car so your uncle would be your loan company. You will make payments to him and he will hold the title until the loan is paid off. He doesn’t need you making a late payment and ruining his credit. He will then make the payments on the unsecured loan (assuming he has liquid cash, he can make those payments on time even if you are late.) The loan will be for the full purchase price (minus deposits) but you can’t deduct the tax credit from that.
You need a WRITTEN CONTRACT! Don’t even think about doing anything related to this transaction without a written and signed agreement with your Uncle.
The Tax Credit will be claimed by your uncle who legally owns the car. This will just reduce his taxes and depending on how much he prepaid on his taxes he may not actually get a check for $7500 and that won’t be until after April 15, 2019. Your contract with him should state that if he successfully gets the 7500 credit then he will apply 7500 to your loan and he will submit an extra principal payment to the unsecured loan company (which won’t lower your payments but will wipe out payments at the end of the loan.)
Not sure what this does to insurance. He might have to get the insurance and list you as a full time driver.
BTW - If he has good credit, he’d probably get a better rate on a secured loan.