Best low cost null voltmeter?

[maginnovision]

So I have been looking for a null voltmeter but I'm finding multiple things. I don't know which are good, which have good value for money, or which are even available as keithley and fluke seem to have many. Is battery powered or AC powered better? Does anyone have a recommendation for a used null voltmeter? I'd prefer to keep cost under $300-400 USD but if that's not possible so we it. I will say I like the idea of the fluke 335D being so flexible in it's use so if it's something like that I would of course pay extra for it. Is the fluke 335D's null voltmeter function comparable to dedicated units?