The space marine contained in that box have been molded in such a way that converting them would be difficult to say the least, converting them to chaos even more so. All really depends on if you want to spend any money doing the conversions or if you just want to do it for free, if your taking the free or extremely cheap route i would go with trying to remove imperial icons and badges with a file and craft knife,paint them with a chaos colour scheme (red corsairs for example) and hand paint on chaos symbols or if you can get your hands on some chaos space marine decals use those(message me and i will mail you some free of charge).
If you were willing to spend money then i would opt for a standard chaos marine squad (10 men) and basicly swap as many parts as you can between the AOBR squad and the chaos marine squad.
pretty much the same for the terminators and commander and dreadnought,
and a shameless self plug, we sell individual bits for chaos marines which can work out cheaper for converting than buying box sets, but the advantage of a box set of chaos marines is he will have the minimum 2 troop units for proper 40k games when the time comes.