BTW, welcome. Since you don't know their state I would change the filters (after testing/unpickling, no need to put in new filters and then run them in dirty water).
As to the spares, how far are you going, and for how long? At a minimum some spare filters and cleaning
supplies, but if you are going far enough you may even want to carry a spare membrane. All depends on your plans and how critical the watermaker will be. For instance, did the PO take out (or never have) fresh water tanks
after installing the watermaker? How much tankage do you have? Could you complete your longest passage
on the tankage you have? If you could not complete the longest passage
on tankage then you are relying on the watermaker to the extent that you should probably be able to do a pretty complete rebuild
, filters) underway. OTOH, if it is a convenience to keep you from having to fill tanks
then you could get by with fewer spares and order them when needed. That's somewhat a personal choice.