While Rudi has explained thigs well, should you decide to pick up this job, you may find it somewhat difficult to approach the authorities here for the extra hours of work you will be putting in. The authorities here will no doubt help you get your overtime but the reality remains that your employer will hit back and send you home should you take that step.
If you are willing to live in shared accommodation it will cost you around QR 1400. You will be spending another 1000 for your food if you decide to eat out. Add another QR 750 for miscellaneous expenses. The rest could be in your pocket each month. It is now up to you to decide.
Lastly, I am very wary of Recruitment Agencies. They may offer you something back home but when you land here, terms of employment usually change and you may end up being a loser. Just be a bit careful.