It is possible to start Smart Home Automation with a small budget, as you are not required to build your Smart Home at once. For example, you can start retrofitting your Master Bedroom and Drawing room initially to understand and learn smart home benefits. Alternatively, you can try lighting automation of the entire house alone initially, and later you can do security automation, kitchen safety automation, etc. step by step to match your need and budget.
However it is recommended to have all the provisions planned and incorporated in the home for a brilliant Smart Home.