1. So the business already does mid to high 6 figures. But is there scope to increase it to a 50 million or even a 500 million $ company? Without knowing more about the industry, we can't answer that. But you need to find that out.

2. The 2 problems you are facing right now is: the code need major revamping. A dedicated sales person is needed.

Because of your background, you seem to be putting emphasis on fixing the code first. Forget that. Make sales the priority. Join as a salesman and try drumming up new sales for the next 3 months. Without putting effort in revamping the code. This will show you:

i. If the demand is actually there in the market

ii. What features are essential and what are not

iii. And if you can drum up new sales, this will give you the money that can be used to fix the code.

3. One client is invoiced about half a million $ a year. This client is an awesome company to bring on board as a partner. Get this client to fund future expansions. Either by pre-paying you for getting discount in price for the next year or 3. Or by giving you a loan. Or by taking an equity stake in the company itself.