Copter for crop sprayer

I’m interested in getting a copter to spray crops. What kind of weight are the most powerful copters capable of carrying? We do a lot of spraying from a 4 gallon backpack sprayer that, when full, weighs about 35 lbs. To be able to spray 4 gallons from a copter would be more than adequate, and we would likely still find it useful if the copter could carry half that and we just refilled more often.

Are there off the shelf copters designed for this purpose? If not, could anyone recommend a frame that I could use as base to build one? Thanks

Are you looking for something like this?

Large Hexacopter (23L)

Large Quad (10L)

Yes, thanks for the links.