I don't know if this discussion is still open but I have burned a few brain cells thinking about this. Admittedly it's only in conceptual stage but this is where I'm at.
1.) The basic idea is to quantify the nature of an Astartes before they get to the level of DW.
2.) An Initiate Space marine (Scout/Bloodclaw/Neophyte) is the equivalent of a RT starting character so ~ 5000 xp.
3.) A full fledged Battle brother is the equivalent of a BC CSM so ~8000 XP.
4.) A Veteran SM is the equivalent of a DW marine.
5.) Space Marines would not have comrades per se. Instead I would recommend purchasing solo and squad modes in a similar manner to comrade xp advances.
6.) As I understand the fluff, All SM start out as fairly generic combat marines and do not actually specialize until they reach the veteran stage. As such there is only one SM "Class". They will progress from Initiate, Devastator, Assault and Tactical Marine assignments in whatever order is dictated by their specific Chapter. Their choices of skills/feats will determine their specialization when they reach Veteran level.
7.) Equipment: SM Initiates start out with either Scout Carapace or Powered armor.
All Space Marines are equipped with a bolt pistol, Astartes combat knife, 3 Astartes Frag grenades and 3 Astartes Krak Grenades.
8.) Advancement: I am no expert at dissecting the aptitude system so I will leave that to others who understand it better.
That's as far as I got for now. hope it helps.