I think you would be fine with the rear rack type depending on the shape of the ground your spraying. the further forward on the rack, the better. If you can get it centered above the rear axle it should not affect the steering and front end weight much at all. I don't think you would have any problem with the rack setup, your not going to be doing hillclimbs with it or fast trail riding with the extra weight there, just puttering around.
Also, the 12v electric type pump is going to be the best setup for the sprayer. I am also thinking that the chemical your talking about is 2-4D (same pronunciation, but it might help you find what your looking for in a store or something). It kills basically everything but grasses.
current rides:
05' 400ex
02' yz85
84' 200es big red
84' 200s with tlr200 engine project (almost done)
84' atc 110
83' 225dx with 340cc kohler sled engine