Void Staff before deathcap is probably not the best idea, any tanks or bruisers won't have as much MR as they would later in their build. Carries that don't build MR will only lose 12 of their innate 30 from a Void Staff, whereas Rabadon's will give you at least 180 AP. Granted VS is cheaper, but still.
Also, Blue buff CDR is 25%, so once you grab Ionian Boots you'll hit the cap w/ blue, and the masteries/runes will be wasted. I know you won't always have blue, but something to keep in mind.
I know like nothing about Malphite so I don't even know what to say about that.
--
http://img.imgcake.com/McBones/MvC3politicalupdatejpgyt.jpg