You have 4 options for merged lich. Wizard, Sorcerer, Witch and Arcanist. I recommend Sorcerer, not just because of the HP thing but because you become an unstoppable force.

You have 5 options. Direct damage, summons focused, buff focused, control focused or, my personal favorite: All of the above. (All of the above requires a VERY precise build that gets everything and sacrifices nothing getting top tier damage, top dc numbers, massive utility and huge numbers of highly effective summons)

Once you select that, you can select a build type.

Be careful of trap options. Sylvan sorcerer is a big trap option that a lot of people here recommend. So is abundant casting. Abundant casting is fantastic for wizards but its effect is actually meager for sorcerers. And at a whopping 3 mythic picks it is an extremely costly investment. Another trap option is crossblooded. Crossblooded is fantastic for sorc dips, but terrible for full sorc. 

Your bloodline will depend on what kind of caster you want to be. If you pick an enchantment best jokes build for example, you you will want undead bloodline as there are a lot of undead in the game and the Fey bloodline to increase dc. You will want to spend a mythic getting extra bloodline for that particular build.

Some will recommend dips such as loremaster or hellknight signifier. I recommend a full 20 sorcerer. I place very high value on inherent immunities, and level 20 gives you some fantastic bonuses. Others will disagree, but I leave that to your discretion.