I don't think you would be able to find shopkeepers who give out items on installments here. I do not recall of any such dealer. The system here is to buy second hand items usually from people who are leaving Qatar and this practice is quite common. There is a second hand furniture market at a place called Najma but personally I am not happy with that place. Other than that, some go for brand new items if they can afford them.
You don't need 3 weeks to think over such matters. You can decide within 3 minutes if you are a good decision maker!