Car inventory is a pain in QB as is anything that is unique or requires repairs or upgrades.

Each car has to be a unique item. When you buy the car use that item and the purchase cost becomes the cost of the car.

Repairs have to be accumulated in an expense account before you add them to the car cost. To do that you have two choices:

1. set up a repairs expense account for each car and book the repair expenses to that account.
2. set up a class for each car, book repair expenses to one repair expense account and identify each repair expense by class.

If I was doing this I would make one dummy expense account called car-repairs (and never post to it), and then sub accounts for each car. Once the car is finished you can rename the account to the name of another car and use it for that one.

So you accumulate the repair expenses.

When you are ready to add the repair costs to the car, bring up the expense account (or run a class report if you are using classes) to find the total of the repair expenses – write that down.
Then bring up inventory adjust, mark it as a value adjustment, select the expense account that holds the repair expenses. Find the item car, leave the qty alone, and add the number you wrote down that is the total of the repair expenses to the current value and enter that total in the new total column.

That adds the costs of repair to the cost of the car. When sold that cost will go to COGS (normally). When sold and the qty is zero, mark the item inactive to get if off the screen.