How do I know which type of storage tank that I need?

Well, there are really two main options here; pressurized tank or non-pressure tank.

The pressure tank will come with heat exchange coils on the inside (2 or 3 depending on style) that will be tied into a closed solar loop and open DHW loop. The additional coil (s) can be used for space heating or auxiliary heating. The drwaback to large pressurized tanks is that they require larger doors when you exceed 200 gallons or so.

The Non-pressure tanks are basically custom built, insulated tubs that can be brought in through the smallest of doors and assembled on location to a volume of up to 4000 gallons. With these tanks, the heat exchangers are submersible copper coils typically and designed to specification for each application. Usually, this type of tank is used for home space heating system or for large DHW systems as a pre-heat buffer tank.