Yes, it is a fairly decent salary depending where you come from. Since, your company is providing accommodation and transportation, you will be spending QR 500 if you cook by yourself or QR 900 if you decide to eat outside. Add another 500 riyals for your miscellaneous expenses and the rest should be in your pocket at the end of the month. Of course all this depends on your style of living. Based on the above information you can judge for yourself if the savings you will have by coming here will help you meet your liabilities.
As for joining another company after completion of contract period, I am unsure of the current rules. But I guess you will be needing NOC from your employer to switch over to a new employer. Let me add that the issuance of NOC or non-issuance of one is the employer's prerogative.
Lastly, I must point out that there is no such currency with the initials QTR. The Qatari Riyal is always written as QR