Had we armed this vessel with the weapons that you suggested,it would've been a corvette.
It is not whether a corvette can do patrolling or not,even an aircraft carrier may patrol the sea,no problemo.
What matters is the implications of these patrolling. An off shore patrolling vessel patrolling and intercepting ships,a foreign ship in particular,won't make news,as they are supposed to do so. On the other hand a corvette or any higher ups doing so indicate that navy or the country is in high alert and them intercepting a foreign vessel will almost certainly make news. In my opinion,its not cost,but Indian navy's need for an OPV that has kept this vessel unarmed.