If you go for an magic damage on-it with nashors, rabadons will offer more raw dps than BotrK, and gunblade will offer far, far more sustain and not much less dps with more damage on q+e+ult.
And with an ap heavy built, runaan's just feels wrong(though it probably still is effective)
Also, Wit'S is most effective if rushed as first item, or used later in the built to supplement your dps with shred and mr, so i'd either built it first or get atleast nashors+guinsoo's first.
That's also because Nashors on-hit mostly works via ap, so you want to get some ap right after nashors, or befor it but that would delay the cdr which i think is very valuable.
If you want a no-ap onhit, replace nashor's with mallet for survivabillity and the slow(works nice with hurricane), or if you jungle with bloodrazor.