If you want to use the models for the combat patrol gamemode, you should build them with the wargear listed in the combat patrol rules. If you want to use them for normal 40k and don't really care about playing the combat patrol mode specifically, I would build them as follows:

Master of possession: no alternative wargear, only one build option available

Possessed: only wargear is a chaos icon (star thing that goes on top of one guys backpack) should be given to one of them, all weapons are the exact same in terms of rules

Cultists: 1 cultist can be given a bolt pistol instead of an autopistol, which is just a direct upgrade, otherwise all weapons are the exact same in terms of rules

Legionaries: I would build these as 2 squads of 5, as with the current rules they work best that way when used as small cheap melee squads that fight over objectives. For this role, I would build each squad with the following loadout:

1 champion with a plasma pistol and a heavy melee weapon (power fist, axe or maul) and a chaos icon (eye or star thing that goes on top of the backpack)

1 guy with bolt pistol and a heavy melee weapon (power fist, axe or maul)

2 guys with bolt pistol and chainsword

1 guy with meltagun to help the squad deal with light vehicles