Hello,
your mixer is an active or a passive device? Take in account the maximum LO level allowed to the mixer. If the DDS selected has good phase noise (or jitter) for your application, you only have to filter out harmonics with a 5th order low-pass filter till 1.2GHz, however, you'll see with this that the second harmonic of 600MHz is still passing through the filter, thus, due to the high bandwidth of synthesis, I would divide it in two bands (600-900 and 900-1200 MHz, for example) and place two parallel filter switched by an RF switch with good isolation (>20dB)
Regards.