As @Deejay Dave says, the RX would support only certain MIDI functions of Serato DJ. It does not act as the audio device, so you would need one capable of "unlocking" the software and routing the audio. It also does not control the playback of the deck (platter/jog) as those items are non-mappable in Serato DJ.
If you want a unit capable of stand-alone performance, the XDJ-RX is designed for just that. It was not designed or marketed to be a MIDI controller.
If you want a controller capable of handling Serato AND/OR rekordbox, but will need a computer present in either situation, the DDJ-SX/2 and DDJ-SZ were made just for that purpose.