Hiya,
I have it in my head that this town just needs a guy to kill weeds on mostly vacant lots at a reasonable price. I am exploring that Idea. If folks don't kill the weeds, they will be paying for lot clearing. I live in the Mohave valley where, of course, it is very hot and humid this time of the year. After our summer monsoon rains, the place is a virtual paradise for weeds. Some are capapble of getting to 6 foot high and some are more like grass, and of course there are some in between. I am not expert at identifying them for you.

I think I can develop a part time business with a simple backpack sprayer, and a resonable price. Am I wrong already? But, I really don't know of the best products for general weed spraying. I want a product that will kill, kill, kill!

I have used Roundup on my own property and it seems to be quite a killer. However I usually dig out the taller, tougher weeds. I want to guarantee my work, say for 6 months like the pest control people do at a similar cost (around 3 cents per square foot.) So, I am looking for suggestions on what products you would recommend for a complete kill, across several species of weeds for mostly vacant lots. I realize when dealing with weed kill around lawns and Gardens, I will have to be more familiar with appropriate products.

Use Ortho Ground Clear and spray early in the growing season, while the weeds are max 2-3 inches. But nothing will grow (grass, trees, flowers) if you use that.

Mojave (or Mohave)? Are we talking California? In California, pest control businesses and applicators must be licensed. A license requires fees, insurance, and knowledge to pass a test. I assume most states require that pest control for hire requires a license. There must be accountability for this work. Contact your local department of agriculture for the regulations.
Once you start studying the rules and techniques, you will learn that there are a number of pre-emergent herbicides and soil sterilants like diuron, monuron, atrazine, bromacil, etc. But depending ont the environmental awareness of your community, herbicides may not be your best option. Appropriate plant selection, cultivation, and mowing may be the methods of choice.

Be careful with any weed killer that claims to clear for 6 months to a year, they migrate in the soil when it rains. You said you get lots of rain and this could kill things not intended to be killed.

I found a spray called ortho weed be gone spray it and within 24 hours you should see the weeds 3/4 way dead however it takes around 2 weeks until aculy completely gone I got mine from wallmart