If all you are going to use it for is glyphosate, there's pretty much no reason to mess with a controller. With a tractor rig that size there should be no problem maintaining a steady speed. Concentrate on learning the rest of the machine and how things work and leave the controller for the next generation.  Simple is good....more complicated becomes a necessary evil  as you go to bigger and faster rigs, but to the maximum extent possible, simple is GOOD>

After looking at the pictures more closely, I will go on to suggest that you take the money you are thinking about spending on a controller and use it to replace the diaphragm valves with ball valves.  Diaphragm valves pose lots of potential for a valve to not open fully and mess up the rate to different sections of the boom.  If it is not severe enough to show up visually, you will not know it is even happening until you see control failure or until the hext time you are checking flow rate across the boom.  They also tend to be a constant headache to keep working.  The ball valves, on the other hand, just work. 

Take the tips off and chuck them. They only cost a couple bucks a piece and they are vital to doing a good job, I strongly recommend that new tips should just be standard pre-season maintenance......which reminds me, it's getting that time of year, I need to order some spray tips.