First thing first, people saying to buy swift dx and dlx over WagonR is just opinion and personal preference, WagonR is better in terms of value for money as i see it. WagonR gives better mileage for me it is giving 15+ km/l with ac in city drive, less registration cost, less annual and withholding tax, better seating space, your knees wont touch the front seats, 1.25 lacs less than dx, Swift is no doubt better car but you have to pay more for the immediate cost as well as cost of running the vehicle.
Now the 2nd thing, i bought WagonR vxl fron Meezan bank with 30% down payment. It is better to pay maximum downpayment as posible. Installment includes mandatory Takaful for 3 years. Just make sure you can have enough to pay monthly payment, quick way to calculate is you should be saving equal to Installment amount*2= total saving, if you are already saving this amount you can easily pay installment, like if your installment is going to be Rs 30000, you should be saving 50-60k monthly today
Further to add, this decision should not be taken by considering the salary, for example you have salary of 300k per month but you consume 270k and save only 30k then opting for installment is not a good idea, in short this decision should be make on the amount you save in month end not on total salary.