[Elecraft] KPA500 and KAT500, befuddlement re: cables

[Joseph Carter]

After 45+ years as a ham I am contemplating adding an amplifier - a  KPA500 and KAT500 to complement my precious K3 and P3.
I have downloaded both manuals and it must be my age catching up with me - I am flummoxed by trying to figure out the cables I need. 
If I understand it, I will need the cable that comes with the KPA500 and the "KPAK3AUX" while not necessary allows complete integration with the K3.
What confuses me is what cables I need to order with the KAT500  - I see "E850863" and "E180980" cables.
Obviously, I want to just place one order and make sure I have all I need but don't want to order anything I don't.
I seem to recall a number of issues discussed on the reflector - did a search of the archives but still don't understand - especially about the "Y" cable.
Love my K3, P3, and KX3 and all I have learned from the group here.
Thanks es 73,
de Joe, w9jc