Mordekaiser is a tanky ability power solo mid. He's a brick wall in the middle lane. He generally harasses and creates so much pressure through pushing that he can steal the opposition's jungle and control the game.

I can't imagine starting with any other item on Mordekaiser other than boots and 3 potions. The movement speed makes you almost ungankable early game, which is Mordekaiser's primary weakness. The movement speed also makes it so ranged champions can't easily stick to you in lane to continue harassment to actually break your shield. The potions are also quite nice for maintaining your health as Mordekaiser can actually have his health poked off before he gets level two and three. The reason for that is the shield generation from his abilities is actually rather low.

Your first base should always include a ward hopefully with your revolver. If you can stay in lane until you have 1275 gold, you're doing perfectly. Your build is pretty on track. A few spare potions regardless even when you have the revolver is quite nice, just in case you get poke a little bit. The next time you base it should be for your Sorcerer's Shoes. Mordekaiser could easily run Ninja Tabi or Mercury's Treads. The main problem however, is that by also doing more damage, he also generates more shield. So in that regard, Sorcerer's Shoes are a defensive item as well.

The decision on whether to buy Hextech Gunblade or Will of the Ancients isn't really clear cut even to me. Hextech Gunblade gives much more damage output, but also costs significantly more. You can turn around and look at what Will of the Ancients actually does for your team. If you're running double AP it's incredibly potent.

Rylai's Crystal Scepter is an incredibly good item on Mordekaiser for two reasons: He lacks crowd control to actually stick to people for multiple rotations of his spells and he really needs the health statistics just so that one crowd control doesn't prevent him from activating all of his spells for the shield generation.

Spam your E to farm, as it will build up your shields to take on any harass your opponent may throw at you. When using your E to farm, try to harass your opponent at the same time while using the skill.

Marks

Mordekaiser's primary damage source is magic. Thus, his only optimal options for marks are magic penetration or armor. The reason I say that armor is a good option is that you can choose Mordekaiser for a safe two versus one at bottom lane against a ranged carry. I've also done some experimentation with Greater Marks of Focus (ability power) to some success. In theory, you generate more shield off of creeps as they have little magic resistance and it also synergizes with Rabadon's Deathcap. However, the drop back is your reduced damage to champions and you'll have much less lane dominance.

Seals

Armor seals are pretty much the best choice, they ensure that you'll survive through a barrage of auto attacks if the enemy plays very aggressively against you. Health per level yellos are very good if you're very confident you'll win lane or against a weak kill lane like Morgana.

Glyphs

As a first choice I would take ability power per level glyphs, as stated previously they are a defense for Mordekaiser. Determining how much shield that is generated by his ability power is a simple equation of using the shield coefficient and multiplying it by his skills ability power coefficients. You can also run 2 Glyphs of Insight for 10 magic penetration and 30 with Sorcerer's Shoes.

Quintessence

Movement speed quintessences are generally the best choice on Mordekaiser, however ability power and health are also good options.

For masteries I take 21/9/0, standard for an AP who doesn't use mana.

Flash is a must. It is the best escape summoner in the game. There's a reason Ghost has been seen a lot less commonly in competitive games and that's because it is just an inherently weaker escape skill. It's a much rarer situation that flashing over a wall won't get you out as opposed to Ghost.

I think the reason that flash is so powerful is that turrets are incredibly strong in League of Legends. You're pretty much always just a flash away and ghost doesn't insure you make it to the turret due to slowing effects.

Ignite is mandatory on Mordekaiser just for the probability increase of getting a Ghost from Children of the Grave (R). Mordekaiser becomes incredibly powerful with the ghost active.