Alright, so lets look over what you will have to start with:
So starting i'd have
8 Khorne Zerks
1 or 2 Rhinos
Depending on what you want to do with the possessed, use them as possessed or as something else, you have access to eight berzerkers, up to twenty chaos marines, or fifteen and five possessed or five chosen. Alongside the Tyrant and a pair of rhino's thats not a bad start of things.
-You want the guy as your starting HQ, so starting HQ unit he is.
10x Marines; 2x meltaguns; aspiring champion; powerfist
7x Berzerkers; skull champion; powerfist
5x Chosen; 3x plasmaguns
With this, you still have five more models left to work with when your army expands in size. A big thing about five hundred point armies is how little you will end up having, because the big thing is to try and maintain some form of speed/protection while cramming as much as you can. (Hell, even with the above seven fifty list, those points are eaten up very quickly. After just getting the basic HQ and two troops, there was less than a hundred points left to work with.)
The above list uses what your going to get, likely with a few liberties in regards to the number of meltaguns/plasmaguns you'll have access to, and gives you a mobile force that is decent in close combat and is capable of packing a punch from the start.
CE5511; what are plauge marines? Are they anything like plague marines or are they just the lesser run-off/shameful past of something else?