1. Check that buffing lower level targets works (with or without spell ranks option)
2. Implement automatic rank lowering (using the spell rank lists)

From here (the BUFFS table) you can see the diff buffs for wotlk along with their minimum levels of the target which they can be cast on (mostly 10lvls below the req level for the spell itself) https://github.com/anzz1/lua-npc-buffer/blob/main/npc_buffer.lua

But I guess if you just cast the spell without a rank it should just choose the right one automatically anyway?

There is a checkbox in options to use unranked spells.
But i really need to look into how that is programmed, that code was not touched in months if not years.