Forum adverts like this one are shown to any user who is not logged in. Join us by filling out a tiny 3 field form and you will get your own, free, dakka user account which gives a good range of benefits to you: No adverts like this in the forums anymore. Times and dates in your local timezone. Full tracking of what you have read so you can skip to your first unread post, easily see what has changed since you last logged in, and easily see what is new at a glance. Email notifications for threads you want to watch closely. Being a part of the oldest wargaming community on the net. If you are already a member then feel free to login now.

I'd like to make an army of MkII or even MkI Thunder Armor space marines, to be from the very first deployments of astartes in the final battles for Terra and the Solar system.

Are there rules which I could choose which might reflect this? A certain legion such as I guess Dark Angels? Are there rules for fielding II / XI / Unknown / Unspecified legion marines? Any units which were fielded earlier than others during the Heresy?

Well nothing on lost legions since they are kept secret with nobody(not even GW) knowing what happened to them. Can't really create rules without giving up on SOMETHING about them when they are supposed to be closed door mysteries.

If you time before mars was conquered or even shortly would probably be good to avoid fancier tech as Mars and it's production wasn't yet on board. Likely marine forces were fairly basic at that time.

I would just stick to the basics, and go with the crusade list book, as legion specific units wouldn't have been formed then. Look at the unit entry comments, as they often give an insight into when they were introduced. The box dreadnoughts, for example, existed before the unification of terra, so they'd be appropriate for you. Typhon siege tanks didn't exist until after Perturabo was found, so you'd know not to include one.

Earliest Crusade (actual Crusade, not Legions running around the Sol system) forces that left the system to crusade were the Dark Angels. They have the whole 'forbidden technology stored away from other Legions' thing going for them as well, part of why they get so much plasma for example.

You'd also likely want more Mk2 than Mk1/Thunder Warrior just from the narrative that the Thunder Warriors were precursors to the Marines and were subsequently wiped out (though nothing stopping you doing Thunder Warriors, but on the modelling project side you're probably not wanting to go past mk2 as the most advanced equipment for the lucky survivors).

And I misread part of your post, but was going to cover this anyway.
All the Legions (all 20) took part in actions across the Sol System. They may have had different names and colours compared to later changes once the Primarchs were found, but from a narrative standpoint there's nothing stopping you modelling a force from any Legion.

Lost Legions are tricky as there's nothing for them. Closest rules wise you get is the comment of select to have either Counter-attack or Stubborn (I think it's pick one of those two, book isn't to hand). to apply to all Astartes in the force, so they could be used to give a 'Legion' rule to the Lost, but there's nothing else to add to them. Unless you go the Blackshield route.

As for equipment - Bolters would probably be less common depending on what you're modelling. They replaced Volkite as the standard legion weapon at one point, so going heavy on Volkite is probably not a bad shout. Especially once Mars was brought in-line.

Just some ramblings hope it helps.

Dark angels would be a great choice, and could probably be adapted as a "counts as" to any of the terran legions, but could be hard for some to swallow.

Blackshields could be a good choice. Pariah armour could be used to represent MKI or even early pre-mars MKII suits.
Xenos Deathlocks could represent some of the esoteric weapons.
Marauders are a bit more "brutal" and could be easier to capture that early terran barbarian theme.

Units wise I guess stick to rhinos, proteus raiders etc. lots of boots on the ground.