I might have misunderstood the intended capacitor function. As described in post #1, you are talking about a series capacitor in the RF feed line, part of a bias T. It should have << 50 ohm RF impedance, respectively doesn't drop RF voltage, only DC. It must be rated for the maximal DC voltage and RF current.
I suggest to sketch a principal schematic and calculate required RF voltage and current ratings.
That's how I read your description
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