I have both an FX5 and FX6, and although I've not unboxed them yet, I would say at most you might want to keep some silicone grease (should be kept available by ALL canister filter users) to lube the seals at each maintenance interval.
Maybe 1 extra set of seals, but I would personally never keep a spare motor(for the price keep a spare "canister filter"), well unless you're using SAND and aren't running a sponge on the intake as a prefilter.
Sand tends to really kill FX5 and 6 due to motor being located at the bottom. I don't like sand anyway (ruined my aquaclear 500's when I tried running with sand 10 years ago, NEVER AGAIN), but thought you might want to know..
Best of luck...