For what purpose was this flat previously used ?

User has to be specific 

The permission given by VMC must be specific 

When you say professional office very many things can come under its umbrella 

So the permission has to specifically state that the user permitted is for a dental clinic 

The society may have an objection because in a dental clinic there would be footfall of people and patients 

Whereas in a professional office as used before there could be a fixed staff of people who would be coming to the office 

So please elaborate a little bit more on the previous user. Professional office is just too general 

Ask seller to obtain NOC from society for purchase of flat 

2) it should be mentioned that premises would be used as dental clinic by purchaser 

3) since society has previously granted permission for use of flat as office premises society would not object to use of flat as dental clinic 

No if it's already a change of user you can use it but you need to seek shop & establishment license in your name. There is no objections on dental clinics in Mumbai anywhere in residential area

You have to gt the property by mentioning its current status i.e., a commercial property.

If the society is objecting then you can obtain no objection based on the registered document mentioning ther nature of property is commercial for the purpose of running yor clinic.

If the society is refusing to issue no objection then you may resort to legal action on this.

If the authorities concerned has sanctioned approval for change of user to the property you propose to buy then the society cannot object to you running the clinic.

If they do so then you can resort to legal action as per provisions of law.

Before purchase of property ask seller to inform society that flat is being purchased by dentist amd would be used for dental clinic 

if society gives NOC for sale of flat to dentist for dental clinic then only go ahead with the deal 

You need to purchase it first. No need for noc from society it's not mandatory. You have right to operate clinic in Maharashtra in residential complexes