Vinal,
Nothing to stop you paying the AP in advance, I also believe that if you do pay a deposit, it shows commitment to the dealer, who may push your order forward, or at least not be tempted to give it to a private buyer, it also gives you leverage if things get a bit long winded. Pay using a credit card,you wont lose anything if the dealer goes bust. The AP has to paid, so you could look at paying by installments. It may also help if you are looking for a discount, off the AP to dangle a deposit.