Are you using the contactor as an Estop Contactor?
You will want a contactor with DC voltage contact ratings. I would run both the DC- and the DC+ across a contact.
Eaton Moeller DIL or XTC series contactors are good, Schneider Telemecanique LC1DT series, etc....
20Amp contact rating would be fine.
Just remember to look at the coil voltage you intend to use. I would recommend 24VDC....
Hope that helps
Marty