3nm is 300mnm just like 300mnm is 30 ncm.
5.5Nm is a reasonable amount of holding torque and if you use the existing belt they should give you enough torque to run the machine, but you need to define what you are using it for to determine how much power you need.

Quote Originally Posted by funkmonkey View Post
Just Looking through a box of old motors I have here at work,

There are three with the following detail

1.8 degree step, Holding Torque 550mNm

2 VDC 2.8A

Are these any good? Noticed that you put down a torque of 3Nm so what is mNm?