Don't hesitate and buy Madred's, it's is surprisingly good on Pantheon.
I was kinda with you till you spewed this non-sense. How is an on hit proc good for a champ who doesn't scale off of AS, and spends the vast majority of their time casting spells now instead of Auto-attacking. I rarely get more than 1-2 auto attacks in between my SS and HSS now that they have much more reasonable CDs... especially if you build a little more CDR.
Personally I feel the new panth is built as a phsyical damage mage, who gets most of his DPS from spamming his spells. To that end, CDR, ARpen, and straight AD are going to be your best bets on increasing your output. His spells do not benift from crit%, crit dam, or AS whatsoever.
In that respect, I like your core items...Brutalizer is a very nice early pick up, and Last Whisper is a great late game pick up if they begin to stack too much AR. I have personally been having great success grabbing an early Glacial Shroud to help me spam my spells more and make me a bit tankier late game when upgrade to Frozen Heart.
I personally have also experimented with Manamune, but i tend to have trouble maxing the mana out, and have had equal success without it as much as with it.
I think a strong choice would be to grab brutalizer, then get your first BFS...and keep tabs on enemy equipment as to whether it would be best to grab Last Wisper, upgrade your BSF, or maybe even just buy another one.
As for MR, I really like Merc Treads with MR/lvl glyphs. If i need more a Hexdrinker is a nice pickup vs many bursty champs who like to nuke you with a combo. Also experimented with Chalice to supplement my mana regen.
Good luck finding a build that works well for you,