first off you would allways want to finish boots straight off for the really heavy dmg boost and can got either straight wramongs or get chain west or negatron cloak b4 warmongs but you need that warmongs for health and also you would pick depending on enemy and how well you do.
also void staff if you meet heavy tank team if its purely squish team woth no mr stacking go for rabbadons.
warmongs counters low hp eaters like darius cho and other true dmg ppl.
magic resist is rlly remarkable vs rumble and such + force of nature means hell alot of suistain on morde.
thornmail you wouldnt wanna finish thornmail off early unless meeting some good ad carry who just nom you.
this is not a go in last build for morde this is a dive and tank and spank build ofc if you end up totaly ****py in a game you would most likely wanna drop thornmail dpending on the enemy and get sumthin likw randuins.
this is teh first and only build most likely il be puttting down such details as this.

http://www.lolking.net/summoner/euw/20913182#profile check my ranked stats as morde 100% win rate on summoners rift ranked 10 games on summoners rift 10-0 win loss
3v3 ranked 3-3 win loss stats on twisted treeline.

care when you farm you would most likely want to hit the minions + enemy champs by thus getting harass and gold out of it.
and since u r a mordekaiser ALLWAYS EXPECT A EARLY GANK trust me done alot of 2v1 at lvl 3-6 and won.
but be careful of a few champs when u play morde top
lee sin can crush you if he is the top laner if he knows how to lee sin and also be careful of long range champs such and cait and kog.