Having recently replaced both cills on my car I can vouch for replacing with new....
I toiled with the idea of trying to repair and or replace with 2nd hand items but to be honest even the ones advertised have rust, chips, dents etc. and it would only have been a matter of time before the problem reoccurred and I wasn't for spending £'s on rust remedial works then prep and paint.... for the problem to return at some point.
I got my sills for £114 each from BMW for what it's worth and I got them within a week of ordering...
Equally so with my new sills they've been fully prep'd, undersealed, guarded, painted then clear coated both inside and outside so no chance of rust returning....
Just my opinion obviously but thought I'd share...
Craig