Hi there JoeJenn06! Regarding your water flow, the minimum that most will say is about 10 - 20 times the water volume, so for a 40 gallon, you are looking at 400gph - 800gph return pump. I would start with that number to locate a return pump that meets the 400gph - 800gph criteria.
Side question: Are you going to have a sump or not?
It looks like you can filter searches on BRS, so I searched for submersible pumps (assuming it would go in your sump) and selected pumps that have a flow rate of 501 - 750 GPH, which gives you 7 options from $58-160.
You'll notice that these have a threaded port (think....water hose). The size/dimension of that port is the size tubing you'll need.
Honestly, if BRS is where you want to order this stuff from, I'd simply call them and ask so they can put the kit all together for you. Otherwise, you can buy the pump and then just visit your local hardware store for correct sizing.
tl/dr: 400gph - 800gph return pump. The loc-line flare nozzle will connect to the modular hose segments and then the modular hose segments will connect to tubing or pvc, which will then be connect to the return pump. Make sure you get the same size nozzle and hose segments.