If you use the serial number thread as a reference it really isn't much of a lottery. You could also get one off Craigslist or eBay with a confirmed firmware version.
Tried that route. Sellers either don't want to give you the serial number, even just the part of it that matters. The confirmed firmware switches are just too pricey to be worth it, in my opinion.
I already have a exploitable switch. It would be nice to find a even lower firmware, but I am mainly trying to bundle as much as possible for as low as possible and then resell what I don't need, because retail prices here are insane.
EDIT:
Just got a good deal on ebay for my 2nd switch, including shipping and ridiculous paypal tax, 320 € total. That's still 50 € cheaper than the average price in my country, so if the firmware is not right, I can still resell for the same price I bought it for or even charge a bit more for profit.