The best advice I can offer is to look past the datasheet numbers for the SM435C and look at what is inside the box. It's a low IF receiver that will be prone to image issues just like the little RSP1A SDR receivers although the SM435C will probably offer >50dB image rejection most of the time.
Imagine having a lab spectrum analyser that always showed a mirror image of all signals at maybe -60dBc that were a few tens of MHz away from the real signals. Then try and imagine what happens when you tune the LO. Some signals are going to move as expected and the image signals will behave differently. Some of their other products use a low IF and they do some tricks in DSP to try and boost the image rejection.
I don't think that is going to be feasible on a real time instrument, so I suspect you will have to grin and bear the image reject issue on the SM435C.
