A multistage pump assembly includes a manifold having an inlet port configured to be coupled to a supply pipe and an outlet port configured to be coupled to a discharge pipe. The inlet and outlet ports define an end suction, radial outlet port configuration. The pump assembly also includes a cartridge having a sleeve and plurality of pumping stages that define a pump stack received within the sleeve. An outer chamber is defined between the pump stack and the sleeve. Each of the pumping stages includes an impeller mounted along a pump shaft that rotates about a rotation axis. The cartridge is aligned horizontally such that the rotation axis is substantially horizontal. An end plate is positioned between the manifold and the cartridge and includes at least one cartridge engagement surface supporting the cartridge. The end plate is removably coupled to the manifold.