When you go to purchase your pressure washer, Make sure you buy a belt driven model. Don't go with anything larger than 4gpm if you will be working in areas which have well systems! Most well systems in my area are "NOT" able to fill a 5 gal. bucket in 1 minute and you have to factor when the pump kicks on. As most everyone knows, When the well pump kicks on, The system is at its least gpm.
A 4 gpm machine is the sweet spot between, Not enough flow for efficient cleaning to the domestic water supply not being able to keep up with the machine.
Almost all belt drive systems will pull from a tank. Most direct drives will not. Or at least the ones I've seen.
As others have already stated, Don't worry about hauling water. In 28 years, I've only come across 1 home which didn't have enough water.
When looking for your trailer, If you can find a "Good Used" enclosed unit, Go for it. If you need to stop at a store on your way to the job or heading back home, Everything is hidden inside. A lot more secure than on an open trailer.
Do keep your positive attitude!!! When I started out, I had a Ford Ranger long bed(with a cap). It had a 2.3L 4cyl. with a 5 speed. Even though it had its limitations as with anything else, It did a wonderful job. It pulled a 5 x 8 trailer all over the place...