Why this item choice?
Well the starting items are fairly self-explanatory. You've got courier, a must have for any team and with the 603 starting gold you can happily purchase it if your team doesn't. If your team does, perhaps go for flying courier for that earlier advantage at the cost of not getting all 4 iron branches (you'll ditch them mostly later). Make sure to keep at least 2 branches to build mek later. There are no mana potions due to lich's lovely sacrifice ability, and I find that for a good early game you don't need too much regen.
The mostly necessary items are what i tend towards building. The phase boots are for that added speed boost as lich is fairly squishy, the mek is a great support item for any team and can be used to keep your fellow heroes alive in ganks and things, sceptre is generally a good item for any hero that it works for but with lich you get added damage on top of the already powerful nuke, ***ualt cuirass is a team support item again which will boost the armour and attack speed of everyone around you making you more survivable and damaging in team fights, and flying courier is self-explanatory.