Monthly basic salary in the sum of QAR 10,000 Net (tax free salary)-- If its's better than what you earn currently - it's OK
1. Company car and expenses (Renault Clio or similar)- v. good.
2. Shared apartment with one European architect provided by the company or cash allowance of QAR 3,500 per month -- bargain for a studio if you prefer to be alone.
3. Mobile telephone expenses paid by the company - v. good
4. Vacation: Three weeks paid leave upon completion of one year service -- good but can be improved. ask for 45 days per year.
5. In case of resignation 45 days written notice is required - sounds standard and seems OK.
6. Workmen Compensation Insurance -- OK- standard, but get a private insurance as well from your home country.
7. All benefits as per Qatar Labor Law-- OK
8. Period: Two years or unlimited contract -- a specific terms is better.
9. Workdays: 6 days a week -- How many hpours per day? -- or how many hours per week? and if beyond that, how are they going to compensate you?
10. Two return air ticket to Madrid per year-- Business Class? :-) but 2 times a year is already OK. Ask then if you can convert it to cash if you opt not to use it.. extra savings.
11. Bonus per year up to 4 months basic salary based on achievements -- I prefer specifics-- but it does sounds good.

How about health care? If you opt to bring your family?

good luck.