The first thing I thought when I saw the model was "Wow, I found my Abbadon model." Indeed, one of the main reasons I have not run Abbadon is because I hate his model. That also means I've never built a chosen-as-troops list. I presume you just sort of maximize damage output. I had the idea in the following way.
First I thought, well you'd have to chaos-ify him a bit. But then it struck me that you could do Abbadon as is, with units of Sons of Horus as chosen, and a unit of justaterin terminators in a land raider proteus (which basically has the same armaments as the CSM LR).
Of course, I still want to include a blob of 30 Cultists and a lord in the backfield to sit on an objective, so for that I'd probably buy 3 squads of Guardsmen, give them 3 flamers, stick a Legion Champion in it.
Abbadon would run with a unit of Justaterin Terminators in the LR Proteus (or a MK IIB). Take 5 chosen squads with rhinos, you could buy the MK IV bundle and some special weapons and assault weapons packs to equip them.