It all depends on how you eat and what u plan to do for 1 month...

Avg meal - 20(KFC, McD etc) to 50/60(good restaurants)
Movie..? 35QR
Phone - depends but avg 100QR if u plan to make calls back to your country.
Internet - again depends - 3g from ur fone is 400 for unlimited, if u get a landline then net is 200QR/month
People can live on 1500/month... but it all will depend on how you spend it.

And yes you can convert INR to QAR at most exchanges.

Your debit card can be used if its international - check with ur bank on the charges.