Asif,

1. for visit visa processing u need a letter from your company, your ID copy and Passport copy of your wife.

2. Visa is extendable upto 6 months.

3. Immediately after arrival should proceed for medical and afterwards only visa can be extended upto 6months.

4. the details are available in www.moi.gov.qa but mostly in arabic.

if u need more info please PM me.