The DC voltage is blocked by other capacitors in the circuit, so the only voltage to be concerned about is the RF voltage. For 50 ohms (matched) that would be 71 Vrms or 100V peak for 100W. Allowing for SWR variations and other 'oops' factors, 500V should be fine. I'd suggest Mouser part number 598-CD10CD120JO3F.
-Tony, K1KP