It really depends. Are you using the pump as a return from the sump, or are you just connecting it to the SCWD? If you are using it as a return from the sump you should try and match the flow rate of your overflow. Plan what you are going to use for plumbing parts and use the flow calculator to see which pump will work best.
Sorry I missed part of your question. You can find almost everything you need to plumb your tank at a hardware store like lowes, Ace, Menards etc. The union ball valve is important as a means of isolating your pump. If there is a problem with it you can close the valve and remove the pump without having to lower the level of your tank. (PS you should also use a coupling so if your pump does go bad you wont have to cut any PVC to change it out)