Well, there are rules for Mannfred on foot, so after I built my own Arkhan, I took the manfred model & re-posed his legs to have him walking. He's... he's kind of huge. The mortarchs are designed in a significantly larger scale than regular warhammer models, which, when taking with the outstretched staff and swirling cape kind of puts him on a 40mm base at a minimum. Additionally, due to the downward angle on the staff (which really does not want to glue in at any other angle, so changing it would involve re-sculpting his arm), I had to put him on top of a built up rock pile of cork board, further increasing his size, so he's a veritable giant next to skeletons and ghouls, and towers over even his progenitor, Vlad.
Still, the conversion works:
More pics in my hobby thread on Carpe Noctem:
http://www.vampirecounts.net/threads/age-of-nagash.29983/#post-436023
So anyway, yeah, that's what I'd recommend for Mannfred. Don't know what to suggest for Neferata.