Do I have to order a complete repair kit or can I just order the part I need from the repair kit?

No, you do not have to order a complete repair kit as individual parts can be ordered separately. The parts that comprise a repair kit can be found in the catalog at: http://www.hydac-na.com/sites/hydac-na/pages/catalogs.aspx > select Accumulators catalogs or search Spare Parts, Seal Kits & Tools from the search bar at the top.