A phono stage/preamp does two things: 1) provide RIAA equalization and 2) provides extra gains for the lower output of cartridges. As a general guideline, you need 30-40db extra gain over line level for high output MM cartridge, 50-60db for high output MC and over 60db for low output MC.
The phono jack of your Marantz should give you RIAA equalization and enough gain for high output MM. As long as you are using a high output MM cartridge, you dont need to buy anything. But if you want to use a low output MM or a MC cartridge, you will need a separate phono preamp and connect its output to a line level jack, usually marked as AUX in, and not to the phono jack.
The phono jack of your Marantz should give you RIAA equalization and enough gain for high output MM. As long as you are using a high output MM cartridge, you dont need to buy anything. But if you want to use a low output MM or a MC cartridge, you will need a separate phono preamp and connect its output to a line level jack, usually marked as AUX in, and not to the phono jack.