Help building cult of the possessed?

So with 8th edition of 40k reigniting my love for chaos and then stumbling back onto this forum and remembering why I love mordheim I kind of want to look at building yet another warband (I love building and converting...) but this time giving myself another crack at a cult of the possessed warband.

I know they're a very difficult one to start with, but I've always had a soft spot for the crazy band of cultists and feel the provide lovely conversion opportunities. I'm hoping to ask for some assistance or recommendations in cool ways to convert and make these models.

Ideally I would like to stick to GW lines, though I get the feeling I may require to look into something like reaper for my magister (I'd actually quite like to make a female one, fancy dress and everything. Gotta have that glamorous servant of chaos feel) though I can always adapt ideas to fit things better. I am however tempted by the frostgrave cultist kit as it looks like a lovely way to make a lot of the henchman and with hoods perfect for the darksouls types. Though I'd have to ask how well they scale up against warhammer fantasy stuff.

It could be quite nice to see what sort of models and conversions others have done for their cult models, provide some nice influence. I'm considering going with a slight tzeentch flavour to my warband, I do love my mages and the like.

Though anyway tldr, what sort of models and kits do people feel work well for a cult of the possessed whether henchman or heroes? And also how do people like to build their warbands? Any particular types to make lots of or avoid? Mutant wise I've often just played around with extra arms which have made interesting models. Though are there other mutations people find really fun to build onto models?

For my Cultists, I mostly went with the Frostgrave Cultist kit. The hood heads are perfect for recreating the original Mordheim Cultists, the undead parts which come with it make for good mutation/possessed features, & it's an easy kit to work with. The choice of heads includes half-hoods with the lower face visible, full hoods, & hoods with attached masks (maybe useful for distinguishing Darksouls), & the zombie heads. Plus, its compatible with the Frostgrave humans & barbarians, opening more bits options. Oh, & there're metal Frostgrave cultist models, some of which are really nice.

For my Possessed, I used a Warhammer Fanatic (the one with his head in the stocks), & he's a bit taller, but not so much that it's a problem. Certainly GW miniatures are... chunkier, so you couldn't use GW arms on a Frostgrave torso, or vice versa, but I find they look good side by side. A unified paint job will do wonders for group cohesion.

Oh, & mutation-wise tentacles are fun & easy to model, & will ruin your opponent's day in-game. They're a very available bit, so I just fixed one where an arm should be, puttied up the gap, & painted it like bruised flesh round the join. Unfortunately I used quite a large metal tentacle (old Chaos Spawn one, I think), & the poor mutant had to be rebased to stop him falling over.

I second Rhydderch's comments on cultists. The Frostgrave box gives you everything you need. The new 40k poxwalkers are potentially useful as mutants or even as Possessed. They carry very little high tech gear and are seriously mutated. Extra arms, tentacles and spines are all clearly represented on the models. You may be able to get some cheaply from someone who's bought the new 40k boxed set.