1. AppThemes does not officially support third party plugins, so we cannot guarantee that all will be compatible with our themes, however yes you can try plugins if you wish. Vantage has advertising widget options available which allow the use of html/text. These can be used in header and at bottom of listings pages (home page, search results, categories, etc).
2. If you are developing a site for a client using the theme, this is acceptable use. You cannot, however, resell the theme itself
3. The default payment gateway built into Vantage is PayPal, which does not support Indian currency, however you can use the bank transfer option for manual payments, or alternatively there are other payment gateway plugins available in the AppThemes marketplace which can be used http://marketplace.appthemes.com/plugins/payment-gateways/ (2checkout.com for example, shows INR is supported https://www.2checkout.com/international-payments/). Thanks.