heres a list I would suggest:
chaos lord with burning brand of skalathrax and 9 space marines in a rhino- good AP3, good bolter fire, good mobility, good survivability. the center of your army. Use them to mow down troops
2 squads of 15 naked cultists- cheap wounds and bullets. flexable with both shooting moving and melee. use them to tarpit up elites like dreadnaughts
as crazy as it might sound, a naked helbrute- the powerfist helps take out the enemy
HQ and the multimelta pops whatever light vehicle they may have at 12" easy.
Id never take a helbrute in a larger game, but here it seems viable. Dont let him get caught up with troops, take out
HQs and vehicles
thats 497 points.
a single vindicator or heldrake also seems very strong.
the thing about 500 pt games is you have to be careful about specializing squads. Its very hard to do. You will want flexible squads like these. I may be totally wrong, especially since ive never played a 500 pt game but...this just seems right. Hope it helped.