One thing to keep in mind is that "works better in cloudy" can still mean it sucks compared to mono.
Lets say its the poly is 13% efficient when new.
Lets age them badly to 8% used.
Now compare that to 22% new mono.
All things equal the mono new is going to smoke the poly even when new with clouds. I think you can see the poly isn't going to look to good being old.
But if you can get them for $100 each I'd go with it if its all your going to probably run there ever.
The problem with buying used panels or poly's only really comes into play if you need to expand it later. The total power is equal to the weakest panel in most setups. So if you add nice new panels later the old ones drag them down to whatever they put out.
Again if this is all that's going to be used there then go for it just buy however many you think you will ever need there at once of the old poly panels.