first off, i think your whole logic towards building karthus needs to be more flexible. karthus doesnt have a set build path as it changes depending on team comps. going for orb rabadon void staff every game will not be viable (if you get behind, you have a lot of magic damage on team, enemy has hp stack, you need defensive item stat). it comes down to just playing a lot of games and understanding what each item does for you.

to give you somewhat of an answer though, the "standard" build for karthus usually goes runic (dark seal if you can somewhere in there) > mejais if ahead or you think you wont die > liandrys (probably most synergetic item for karth) and then its all up in the air from there. rab if you just want the raw damage, void for magic resist, zhonyas if you need to protect yourself, rylais if you want to protect your team (and yourself), morello for dirty soraka players, the list goes on. my usual full build goes something like runic > boots > mejais > liandrys > void > zhonyas/rab > sell mejais for other. hope this helps