Before I would spend a dime, I would go out with a crew that washes trucks that is within a few hours of where you live or whatever you can do to see if you even like washing trucks. Lots of people get into pressure washing every year and then a lot decide they don't like the work, hours, getting dirty, muddy, sprayed with chemicals when the wind direction changes quickly, working all different hours based on the jobs, etc....
After you did that, you will know one or two ways that the company you went out with washes trucks and you can modify what they do, copy it or look for something else but this way you can see what they do, how efficient they are, what level of quality they offer, etc....
Then I would start calling vendors and see what they offer, what prices, if they have good soaps and brighteners (many, many vendors don't have good soaps and don't just take their word for it as they will lie to you to sell you anything to make a buck) but then there are some good vendors out there also like Russ at Southside, Brad at Midwest Cleaning Solutions, Blaine at Bioclean Systems that have lots of knowledge, can help you with systems and offer great chemicals.
Like the guys mentioned, there are many ways to wash, soap through the pump, bucket/brush/rinse, 2-step, downstream with switches/solenoids, switch at the gun, etc.....
It would really suck to go and spend money on a system that you had no idea of what it was, no experience in that system or worse, you listened to a vendor that was not only full of hot air but had no experience in truck washing in his/her life to talk about but they sure will sell you exactly what you need with no clue of what you are washing, what system you have/will buy or what experience you have.
Everyone has opinions and there is a lot of great knowledge out there, just be careful of who you talk to and don't spend a dime before talking to others out there.
Remember that the local vendor probably has good shop skills to fix your machine but might not want to work on it if you did not buy it from him, watch out for those guys as they will hold a grudge against you because they did not sell you the over-priced rig and watered-down chemicals to go with it for your new venture.
Don't believe everything you hear out there as there is some great info but a lot of mis-information, especially when it comes to environmental cleaning and issues.
Good luck.