I advise to go with a 220V system as they draw less amps. The only advantage of a 110V system is powering it with a cheaper genset.
For drain-back, there are specialized systems available for cottages.
Most all tanks use a Tee of some sort. Some tanks may have a separate in and an out so the Tee is part of the tank. With the exception of a sand separator, in-line filers should always be placed after the tank.