I have no idea. I wont' be paying for it in all probability. So I have no idea what factions and classes are currently planned to be available for free. I remember when the game first started to be talked about one of the devs said something about only orks being free to play. But now apparently there are going to be a small group of classes for each. I have no idea. 

If I had to choose. Maybe a dire avenger. I can assume thst would be free to play from the elder side. Maybe a devastator marine or an ork shoota. it all depends on how much a free account would be permitted.