Warning: I know absolutely nothing about the Avatar book or what Nicodem's model is supposed to look like. You said "Avatar of Decay" and "50mm base", so I am thinking its got to be some kind of big and nasty undead thing. Here we go...
This is great for a bloated, nasty look. Not sure how big this guy is, but Reaper models at $10.00+ tend to be at least large enough for 40mm bases.
http://www.reapermini.com/OnlineStore/ghoul/latest/02718
This one doesn't look even remotely Stem-Punk-ish, but the model is awesome! Maybe with some modeling attention, it could be made to be more....suitable.
http://www.reapermini.com/OnlineStore/undead%20lord/sku-down/30010#detail/IG_373_1
How about a werewolf zombie?
http://www.reapermini.com/OnlineStore/zombie/sku-down/02791#detail/02791_Zwerewolfprofile_af
More of a specter look on this one.
http://www.reapermini.com/OnlineStore/undead/sku-down/14182#detail/14182_p_DT
This thing is the epitome of decay, although it will need some serious modeling on the base to make it large enough for a 50mm.
http://www.reapermini.com/OnlineStore/undead/sku-down/03023#detail/IG_308_1
Not sure how big this one is either, but its worth a look.
http://www.reapermini.com/OnlineStore/undead/sku-down/02680
The chainmail puts this one out of place, but tis an awesome piece!
http://www.reapermini.com/OnlineStore/undead/sku-down/02804#detail/02804_cs
A little Cryx for you PP lovers...
http://privateerpress.com/warmachine/gallery/cryx/warcasters/lich-lord-asphyxious
This thing is incredibly cool, but you'll have to pay the price
http://privateerpress.com/warmachine/gallery/cryx/warcasters/lich-lord-terminus
This might be a bit too mechanical for even Nicodem, but..
http://privateerpress.com/warmachine/gallery/cryx/warjacks/reaper
Anyway...that's all I got. There's a ton of stuff out there. Hope I helped.