Issues with frequency conversion
For readers new to the topic: This device doesn't need to be invented. It's called a transverter, and its central component is a mixer. 

You'd drive it with the difference frequency (i.e. (650-520) MHz = 130 MHz), and filter out the unwanted sum frequency around (650+130) MHz = 780 MHz with a simple low-pass that has its cutoff somewhere above (520 + 50/2) MHz = 545 MHz and has sufficient attenuation at (780 - 50/2) MHz = 755 MHz.

I don't see why you'd first mix up; the transition width of 210 MHz should allow for picking good enough "classical" LC/RC filters. (Gut feeling: 9-pole passive Chebyshev LC (that's 4 stages) should do this, and might still be "manageably complex" at these frequencies, i.e. possible to manually fine-tune if parasitic effects aren't eliminateable).

If you decide that e.g. a three or four stage RLC doesn't attenuate enough (and "enough" is up to your definition! You didn't define that in your question.), then, indeed, find a higher IF that you can mix onto, at which there's an SAW filter of required quality, and then mix down. But: this would require multiple oscillators, and each of these will introduce spurs into your system.

In essence: every solution begins with a precise description of your problem, in your case, which flatness in passband, which attenuation in stopband, which circuit complexity, temperature stability, and cost effectiveness you need to achieve.