I think engineering degree ( 4 year ) or polytechnic diploma ( 3 year ) should be fine to get a Family VISA.

in addition final degree or Diploma certificate and Marriage certificate has to be attested from India is required you may find more information for attestation in Qatarliving itself.

Diploma in mechanical must be a Three year diploma from the State board. I think that should be fine. couple of my friends are diploma holders only and they are having family here. However it is up to his company also as they have to provide NOC certificate.