How much buffer room is required for a varistor?
Idealy they should withstand surges up to the clamping voltage (where the varistor redirects all the current to ground). But for a 275VAC varistor (suitable for 220-250VAC), the clamping voltage is quiet high (600 to 750V) and you may not find all parts at or above these voltages, at least not at a reasonable price. 

However varistors do reduce voltage under the clamping voltage, but at higher voltage than the rated voltage. A 275VAC varistor will reduce volatge from 300 or 350VAC but there is no precise limit and no precise resulting voltage which will be always higher. It's gradual up to the clamping voltage. So it's safe to have components rated 400 to 600VAC thought of course the higher the better. 

Components with lower rating (let's say between 250 and 300VAC) will be also protected by the varistor in case of high surge when the clamping voltage is reached almost instantly (within nanoseconds). So in any case, the varistor will be an effective protection.