It’s definitely good to have a plan. And, I agree with you in asking the pros on here for feedback. I have done the same thing. I’m no pro by any means - guys that have replied and others like Racer, Squid, IBS, and others have experience in the industry and are willing to help those that have done their research.
That said, I would echo others in saying do not buy that machine. I am one of those on here that swears my 4gmp/4200psi machine spins my 20" cleaner like a champ, BUT my machine is belt driven. I don’t know that I would try the same setup with a direct drive pump. Additionally, I don’t know that I would go into business (unless you are getting water straight fed from a homeowner’s residential water spigot under pressure) without a belt driven machine. If you were to setup to use a buffer tank, the belt drive primes the machine, pulling the water in so a regular gravity feed from the tank works. Also, the 20" surface cleaner I use is a GP Hammerhead w/ a greasable zerk. Around $400. Hope this helps.