We need to get a idea of how much spending is usually done & in which areas.

It completely depends on your planning . Major cost is Food and Venue (This year sit was around 18) All other cost is not big.

We need to know more details about handling account, do we have some one(accountant?) who already takes care of this for PSSI/PyCon India

PSSI has treasurer who take care of accounting and audit with Income tax department.

But PyCon India budget planning is done be Chair(lead) and Co-Chair(Supporting lead) they both pretty much make final call for that year.