Presumptive taxation for businesses is covered under section 44AD of the income tax act. Any business which has a turnover of less than Rs 2 crore can opt to be taxed presumptive. They must declare profits of 8% for non-digital transactions or 6% for digital transactions, whichever one is applicable.
The GST credit paid on purchase of car cannot be claimed, unless you are in the business of buying selling cars and/or travel business.