The WL Sonnox plugins are basic versions but depending on the source material the results can be good. I’ve tried a demo of RX (3 at the time) and it probably is the Rolls Royce in that price category, but you may ultimately get more value out of upgrading WL (because you’d also then have WL8.5 and everything that comes with that). Don’t forget as well that the Sonnox plugins are also in WaveLab Elements 8, however there’s no “downgrade” path from WL6 to WLE8, but then WLE8 is not expensive; you might find it useful to try the trial. One other advantage of WL is that there is a way to use the WL Sonnox VST3 plugins in Cubase (I don’t have Nuendo, but it’s surely the same).
One thing I will say is that in my experience, simply putting a declicker across the master buss generally does not give good results, and whereas there’s always the “wow!” factor, careful listening usually reveals too much has been removed from the parts that don’t contain clicks. There are some restoration experts over in the WaveLab forum, however, who would have both and thus be better able to advise on the choice, and of course it also depends on how much material you are expecting to process, whether the higher-cost solutions will pay for themselves.