Actually India trained the irregular Mukti Bahinis. These were mostly students, professionals, farmers etc. India trained another group known as "Mujib Bahini",a creation of RAW, these are the Awami League cadres. Mujib Bahini was actually used to put down insurgency in Mizoram. The regular forces had professional soldiers somewhere between 10-20k ( if someone has an exact number, please post it). Which were increased to about 30k.
As far as occupation goes, I doubt it would be easy for India. BD is full of rivers and canals. Logistics can be a nightmare during the monsoon seasons. Back in those days there were barely any bridges, and most were destroyed.
I really doubt India had any such intentions. Taking on 65-70 million people just doesn't make any sense. Also even with the treaty with USSR, I doubt USSR would let India to make that move, the USSR is not going to underwrite joy rides. Friendship treatys has its perks and also limitations. BD had one of those with India. Basically big brother dictates the terms.
India hauled away all the military equipment as spoils of war. I don't want to get into other stuffs that were hauled away too. There was nothing to gain other than a country full of people ravaged by a war and totally pissed off.