Hi,
like with vintage 1176s, you probably won't find two old 421s that are alike enough to make a good stereo pair. And probably half of the old 421s on the market are f...ed, that's true. But if you get a good one,
yeaaahhh...
One way to accumulate a nice collection is to buy fifteen on ebay, pick out the ones you like and re-sell the rest (I admit, that's easier in Germany than anywhere else).
I got six 421's that way, and have them all numbered - one is great on kick drum, two are good on 14-18" toms, two are good on 10-14" toms, and one is wonderful on guitar cabinets. They're all the old beige-coloured type with weird large or small Tuchel connectors.
Cheers, Tom