Not 100% sure on this but I would say that there are two transactions here (assuming this is not a lease):
1. Purchase of vehicle - DR fixed asset, CR Liability account with face value of purchase + GST,, DR GST with GST payable
2. Assumption of debt + accrued interest to be expensed - Monthly: pay down debt with blended mix of Liability and Interest expense (ideally calculate of reducing balance method to front load the interest cost - run a spreadsheet for this or consult a mortgage table)
Alternative for 1: Defer GST of capital purchase and claim GST over the 60 months pro rata to the payments made (to me does not make sense, but there may be other opinions on this)
Note: If in fact this is an interest-free financing, then you should be calculating the implicit amount of interest buried in the purchase price (chose an arm's length rate apply on a reducing balance method) and reducing the cost in #1 by that amount, setting aside as a deferred expense that amount, then releasing it to expense monthly as per #2.
So, as usual\, accountants make what seems to be a simple transaction a nightmare.!
My apologies and hope this helps.
