Yes, 40 GBP is about 55 USD. It seems there are many sprayers in that price range, but what would you expect a quality sprayer to cost?
Do you have any thoughts on metal vs. plastic tanks?
Most are HDPE, (milkjug) plastic, same as the container of most chemicals we buy, and that is a big deal. Acids eat metal, and salts corrode metal. A lawnsman is likely to combine fertilizing salts with a herbicidal hormone. It would be the acidic (sulfur for example) or salt problem that affects metal tips, valves and dissolvable parts. The only thing I have found useful is to stay away from odd tank connections where an extended custom hose cannot be readily added with something like stainless clamp. I have bought the best in store and the cheapest and they all fail. Those stores sell consumables. The best I have done is maintainence them with the seal kits. Hoses and sprayers are available separately. Typically though, and this is big, the spray droplet pattern is too fine for preventing herbicide drift in any wind. This is a difference between commercial and resi grade.
The volumes moved in an open non misting tip will usually require a bigger tank, motor etc. Its hard to get coverage from a low volume low pressure hand held tank with that kind of tip. There are some self pumping pull cart and back pack sprayers that might do however.