Q.1. Yes you have to attest your degree certificates in your country at the Ministry of Foreign Affairs. Then at the Qatar Embassy in your country.
Q.2. Yes you have to undergo medical check at the Medical Commission here in Qatar. It's chest x-ray and laboratory test of your stool, urine and blood.
Q.3. Currently the family visa is suspended for army personnel.
Your salary of 9000 riyals fall short of the required
minimum salary for family visa. Frankly speaking your
salary package is barely enough to live in a family
status. You won't save anything. Good luck.