canister filters are always rated to a tank size... so that is easy, though always find one rated above your tank. My 125 had an eheim pro3 2080 running on it.
As far as a sump goes, i think the rule of thumb is: as big as you can fit under the tank. I don't think you can over-do a sump. More water the merrier. I would say for that size tank, 30-50 gallon minimum but other than that you could go as large as you want.
Im far from an expert so maybe others will correct this post but thats how i would look at it if i was putting a sump on my 125
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