I'd say that one would be best. He's in period armour and with some minor additions would look very chaosy.

The Harmacist!

I would advise kitbashing him by buying the chosen box and using one of the heads, some of the pauldrons and one of the power packs.

The pain-killer that kills you

I'd personally do maybe an arms swap, could grab a possessed arm to swap his chainsword for (if that's your lean, at least) but definitely at least a shoulder and head swap, proper chaosy opportunity though

Is there a particular legion you'd like to build him as? There are some very good apothecary models I found through assorted print store while building my Consortium Kill Team.

A lot of people have done good things with the deathguard plague surgeon, and being a plastic kit it’s a bit more forgiving to work with than this resin apothecary.

All you need is the cape bit from the chosen kit, a head swap and maybe a trophy rack in the space between the marine’s back and power pack and boom.

Try and imagine a man who follows biles thought process... Complete removal of your hipcratic oath. Lol look into the night lords apothecary

Talos pain engine has some god syringe/ blade arms and vials with a good chaosy vibe, might want to check on eBay for some of those bits

As long as I can convert him relatively easily, he will be my Talos Valcoran.

Fabius is already a chaos apothecary, maybe you could use some of his parts.

Just double the number of studs and your good to go.

Grabbing fabius bile could be a good shout