Imo, Rabadon is good only when u have 3 high ap items + 3 skills with high ap scalings.
Kayle + ur build doesn't qualify for Rabadon.
Infinity Edge scaling is way better than Rabadon because autoes scale with %100 ad + don't have cd + can't miss.
Kayle is an auto attacker styled champ + her spells scale with ad + her waves crit so it's better to scrap that Rabadon for gunblade which gives u good mixed stats + sustain + reliable cc with good damage.
Tenacity From mercy is a must. U can take the attack speed or life steal rune if the enemy comp is not cc heavy but merc is a must.
Runnan might be good since Riot has increased her range with the recent nerf but my point is that she is now worthless and heavily gutted but if I am going to play her I will get reaver instead of runnan for more crit damage + mana + max cdr.
All of the above are just my opinions. I might be wrong but I feel that was the best build on her.
R.I.P Kayle...