I have a Solo, also. I like it...but have done a couple mods on it. I put a much better brass metal sprayer on it, (better than the el cheapo plastic one that came with it), and rigged up a holder for the sprayer out of a piece of PVC on the top of the tank.
I also spray about 50 gallons through it a year and it does the job. Solo has 2 types of these sprayers. One has the pump mechanism designed for liquids only, the other pump mechanism is designed for powders/solids mixed in with water for spraying. That mechanism will take more abrasive compounds. The big O ring drops out of the lid and that's what makes mine leak on occasion. Have to watch that.
You just can't beat a good backpack sprayer. You can get into places that a tractor or 4 wheeler can't go. I also use a dye with my Roundup.
I did not know that Stihl made sprayers either, and will look them over, also. I wonder what they cost?