It is quite dependent on the person you are buying from and their need to sell, I have had people that won't budge on their price yet offer relaxed payment plans and I have had people that have come down on their price to get a one payment deal- Both are helpful to me but I haven't had anyone come down on price and accept a payment plan.
The combination would be great but as I said not happened to me yet, I recently spoke to a seller about this same thing Emi, I thought his price was off yet even at a few hundred lower I was still thinking payment plan, he offered a payment plan but wouldn't lower his price - yet no harm in asking - I would always lay your cards on the table first and see what happens.