How close do you want to be to the linked picture?There are a few things that aren't stock. Like the cloak going over his shoulders, the dragon decoration on his helmet and his chest.
It looks like he is wearing mark III leg armor and also a mark III helmet with decorated faceplate and roman crest.
His chest seems to be a strange one, i would say a mark III but with the hanging armor plates attached to the belt and not going over it.
And then there is his spear.
What you'd have to do if you could get hold of a mark III model (forgeworld?) you still have to attach the cloak and add the decorations as well as rework the torso slightly. All of that would require decent conversion skills as well as
GS experience.
You could use this for the crest if you cut it down a bit:
http://www.anvilindustry.co.uk/index.php?route=product/product&path=77_66&product_id=57 or use the helmet from the forgeworld upgrade set:
http://www.forgeworld.co.uk/The_Horus_Heresy/Legiones_Astartes/Space_Marine_Legion_Army_List/Legion_Troops/Legion_Tactical_Squad/LEGION_MkIII_COMMAND_UPGRADE_SET.html The spear could be converted useing a grey knight halberd and some other bitz (a plasma gun/-pistol?)
if you want to have a cloak going over the shoulders you need to convert it yourself. There is a nice tutorial on this here:
http://www.fromthewarp.blogspot.de/2012/06/how-to-sculpt-greenstuff-cloaks-and.html But it requires some
GS skills.
The smaller decorations like the faceplate would require the most
GS skills and aren't easy to do. (Or try to paint them on?)
You have to decide for yourself if it is worth the effort and how close you want him to be - or find someone to commission who can pull something like that off.