Reverse charge case for medical professional with other income on reverse charge

Doctors are having professional income from medical services. They also earn rent. GST is applicable. Since their professional receipts are more than 20 lacs ( professional and rental)they are liable to take GST registration. Now my question is in income tax they are taking benefits of 44ada and not maintaining books of accounts. How will they have to deal for purchases under reverse charge for expenses done on medical services from unregistered persons.