I was just at my regular aquarium shop yesterday and he advised me to go for 1 size up over the recommended chiller size.
But as you know, I would be getting used items and it seems RA680s are more readily available than the RA240. Anyway, I have confirmed the RA680 from a bro here and he gave me a good price. As for condition, I don't know, only that it's 2 years old. Anyway, I think I will just get it serviced before using. Decided to get a used Teco over a new resun as my aquarium shop recommended Teco and Artica. The shop has been running an Artica pn their planted display tank since I was in the Army (10 years ago). Yesterday, the same Artica is there, running without a hitch