Obliterators would probably be best. All you need do is cover them in stringy greenstuff and paint it fleshy (although I prefer the look of the 'flesh' when it's painted as metal, looking more like mutated armour - it's called fleshmetal, after all). Mutilators are useless and Chaos Terminators would require you to get bits that you don't get from
DV.
Since you're new, I'm assuming you're new to
40k as well. If not, stick a bunch of random guns onto both of their hands (in a mass of
GS fleshy goodness, of course). Do you have any people you could get some free bits from? A small Tyranid jaw on a Terminator helmet would add some cool "Chaos mutant" aesthetic to a model, for example.
EDIT: There are also
quite a few tutorials about this in the top results on Google.
One last thing. For my Alpha Legion, I regularly strip Loyalist models of their iconography with a hobby knife. Trust me -
cut away from yourself.