Boots + 3 pots or Doran's Shield. I've found the Doran's Shield start to be extremely effective vs less mobile champs, or in case you don't feel like taking any damage at all. Doran's shield has enough regen that you don't have to worry about killing yourself, and the armor and hp effectively makes you unkillable with your shield for a few levels. If I'm completely shutting my lane down I'll skip boots and go straight for my regular WOTA -> Rylais -> Rabadons/Abyssal -> Rabadon's/Abyssal/Randuins (if you REALLY need armor). Boots are merc treads if they have a lot of CC or sorc boots if they have CC I don't care about. The reason you can afford to skip boots on Morde is that he's one of the best 1v2 champs in the game, with his AOE and shield, and also the ability to burst down someone to get a ghost.
My philosophy with Morde is that it doesn't matter how strong your shield is if you have a whole effin lot of it. That's why I build AP over resistances so through my damage I can keep my shield up constantly, and it doesn't matter how fast it's being taken down in a fight. Rylais is 100% core along with WOTA for the slow and HP.
He can mid or top equally well, with mid honestly the easier option due to the less threat that enemy mages pose early game, as opposed to mobile top AD burst champs, which can really be hard to deal with.
@Journier: If you were to go that route I'd grab a Mejais just to keep your snowball rolling once you run out of levels. The base damage on a solo target with Q is ridiculously high (like 400-500), and very spammable, and the base damage on E got buffed in the last patch.
Fun fact: Morde now gains more shield off of champions than he did pre-nerf = stronger teamfight/late game Morde. Mid game you can get full shield from a minion wave anyway.
I go 21/9/0. Full AP route in offense tree + executioner (rapes with ult), and in defense it's either 3 points in armor or 3 points in mres, depending on which lane I'm taking. Generally Mres mid, armor top. Mpen reds, armor yellows, ap/lvl blues. 3 Flat AP quints. Yellows are flexible on whatever defensive stat you want, but armor or hp per level are the most effective in that slot.
E-Q-E-W max R -> E -> Q -> W