Ok, I'll share a rather embarrassing story here, hopefully as a lesson to others in this situation:
Until last spring, I was renting a 3.5gpm machine from the rental place across the street from me. My power washing jobs were so few and far between, (and I was so busy washing windows) that this seemed workable, and I'd just save up for a really good unit over time.
First job of last year, I call the rental place the day before the job is scheduled, and 'my' machine isn't available. They've rented it out. So I call another rental place, and they tell me they've got a 3.5gpm unit. Ok, great.
I pick it up, and it turns out to be a 2.5. Oh, well. I go to the job, which is a little 1,500sqft single story house, plus a tiny shed. Washing that home was torture, and I felt so awkward. I was just grateful that it wasn't a bigger job.
I finished the job, went home, talked to my wife, and after some careful consideration put in an order for a real machine. Maxed out a credit card to do it. The card was paid off in a couple months.
Moral of the story: if you're serious about this as a business, don't be so risk averse that you slit your own throat by using improper equipment.
I personally hate debt. And credit cards are straight from the devil himself, lol. But in some circumstances, they're worth it.