Good Day Sir,
Here are the answer of your following Questions:
1. You are required to pay GST on rent, if your aggregate turnover exceeds from Rs.20Lakhs and in your case aggregate turnover exceeds from Rs.20Lakh
2. Rent is a part of service and service provider has not opt composition scheme, so in your case you are liable for taking registration in regular scheme
3. Composition scheme limit is Rs. 1.5cr., but in your you can't opt composition scheme, because you have income from rent also.
4. If you have any closing stock, and you switch from composition scheme to regular scheme, then you are eligible to take input tax credit by filing GST ITC-01