For a tank Mordekaiser you need high defense and sustainability. All of Mord's abilities take health, and the damage he outputs generates a passive shield that gradually degrades. With that in mind it is important to have high hlth regen, and some spell vamp. Warmog's will help with regen and as you get more health this regen increase becomes higher. Spell vamp is also needed as it returns some of your health when you cast on enemies. Obviously I shouldn't have to explain the high defense part as it is pretty self-explanatory. If you feel that you are being over-focused or if you don't farm fast enough for your build, I recommend getting thornmail for return damage when hit.

*****This build IS expensive and slightly DIFFICULT to run*******

Now for the summoner spells I prefer Heal and Ignite. Heal is good for early sustain especially if you are soloing. Ignite is not optional. You MUST choose ignite. Whenever you go to use your ult and steal another champ's soul it is best to stack ignite with that, so they have less chance to flee. Sometimes your ult alone isn't nearly enough.