Yes, it is very important to get a license as most of the cities/states in the US require a Real Estate Business License to operate.
It totally depends on you, your interests, passion, and knowledge. There are different types of professions like an agent or a broker, property manager, etc. Choose the profession carefully.
A big Yes, from my side. If you are focused and dedicated towards your work and if you are passionate about it you'll surely build a successful career in this field. You can build wealth, supplement you income & can make a successful career.
I personally recommend having a website. As you know in today's world having a digital appearance can make a lot of difference. It'll help you get more business and help you to expand without any limitations.
There's no exact amount I can tell. But as per my estimation, you need an amount of $5000 - $10,000. It is only a rough estimate the actual amount can be a little high or less.