I worked building maintenance for a metropolitan transit system after driving for them for twenty seven years. I found out they were paying a private company $60K to spray for weed control and fertilizer. I told them we could do it on our own for less but it would require an upfront investment in equipment and getting a couple people certified, it worked. We bought our equipment from a local supplier and never had any problems with those backpack units. We bought spray units that had the tanks, pumps and motor all on a unit we could load and unload on trucks we used to plow lots, it was slick. As an added bonus while doing the spray work we got the next highest rating pay, it was a sweet deal. I later talked them into sending me off to a arborist class on tree trimming so all of the trees were also included in the work that paid about $5 an hour more and before long we just negotiated a separate classification for the folks that did that specialized work.
I found that you pay for what you get, your not going to get very good luck trying to pinch pennies getting stuff from harbor freight, Honda Motors are the best, bar none. The other deal is you have to keep the stuff clean, rinsed, follow procedures when it comes to safety. No guessing when it comes to mixing, buying your chemicals from the state approved agency helps as well, that is where you need to be certified. Friends would ask me if I would spray their property and I told them that I was only certified to spray our commercial property, not residential.