Long term car rental makes sense only if it's a small car. One of these will take you back
approx 1400-1600 QAR/month, which is what you get from the Company as a transport allowance. I know people who's done this for years.
If you buy a Ford Explorer with a car loan from a bank ('lease' it, as they say in Europe), you'll pay may be 3500 QAR/month (approx 1000 USD) and it'll be yours in 3 years.
You do the math!
Cheers!
H.L.