Use capacitors with a minimum voltage rating of 50 volts. A higher voltage rating won't hurt, but size and/or cost may become a factor. You don't want to try stuffing soup-can sized caps in there. Tolerance is not particularly critical, 5-10% is fine. BTW, there is no advantage (and some disadvantage) to using audiophile caps in the high voltage multiplier. If they need replacing, use the same type as original.