I had a similar opportunity that kind of landed in my lap. 10 years later, I retired. Yay!
Like you, I started with the idea that I'd work this one job as long as it lasted, and then see what happened. i just kept getting more business. I used the idea that I'd set things up so I could grow/expand as necessary, but if I had to do it over again, I would have just done an LLC, filing as an S-corp.
Because my work was consulting, with just me, I couldn't work more than a few jobs at once, so I just used Quicken (already was using it for my personal accounts, so I just created a new file for the business). I never bothered setting up Quickbooks, although everyone tried to sell me on the necessity of it.
For timekeeping, I used Excel, and for invoicing I used Excel. everything was easy and straightforward (and cheap).
I think you'll need a bank account and a separate credit card that you use ONLY for business.
I set up as an s-corp, so I had to do payroll. At first, I did it all manually through my bank, but eventually used a payroll company (Paychex) to cut me a check each month and do the corporate taxes. I also used them to administer my 401K.
Good luck!
