Sorry to have slightly different ideas, LDH. My suggestions: Use reducer bushing at the tank outlet to install a 3/4" hose barb, then install an inline filter to protect the pump and reel, then use 3/4 tubing to enter the pump.
On the outlet side you need a 3/4 inch pipe cross. Fluid enters one side. Opening 2 connects to the reel (you need a reducer to accomodate the half-inch reel hose.) Opening 3 connects to the pressure relief valve. Opening 4 connects to the bypass (agitation) hose which in turn tees into the tube downstream from the pressure relief valve. The bypass hose allows you to start with the pump not under load and allows you to direct all the output to the tank for agitation, when needed. You were planning on an agitation nozzle in the bottom of the tank, were you not?
Notice that the pump has a cast iron body--not recommended for Roundup, which is acidic, I guess, (probably short periods of time are OK. )
Notice that at factory new condition, at 150 lbs pressure you only get 5 gals per minute output, and it takes more power. Keep in mid you lose about 10 percent for each 100 feet due to friction in the hose.
Not all roller pumps use a pressure relief valve--maybe someone else can comment on whether or not one is needed. Perhaps you just need to close the agitation valve part way.
If you mount the motor and pump under the reel--which is fine--be sure to put a roof or cover over the pump. You don't want herbicide and fertilizer dripping off the wet hose onto the pump and your brand new motor--nor rain.
Be sure to accomodate how to add, check and drain oil for changes. How to fill gas. And how to pull the starter rope. And clearance to change worn pump rollers.
With my setup for easier rope pulls, I used a pulley attached to the reel; this allowed me to use a longer starter rope and brought the rubber handle up to just over waist high.
You want an air gap fill tube on top of your tank for back flow prevention. Required by law in some areas.