Raven Guard would be the best way to go, I'd think - Ultramarines and IFs are also somewhat similar, but not entirely. If you're going to include Heresy and Corvus armour though, you'd have to be doing post-Dropsite RG to do it in a fluffy manner, since
Delieverance Lost goes into detail about how the RG not only invented Mark V suits (before a standardised version was put into production by the Mechanicum, anyway), but were also the first Legion issued with Corvus armour due to their role in testing the prototypes in the later Crusade. That would be a great excuse to use the unique Corvus bits in the Raven Guard upgrade kit, although I'd limit Corvus specifically to Vanguard Vets (no jump packs? I forget if they had them...) to reflect the Raptor specialists described in that book (whether you'd want to do them pre- or post-mutation or not would be up to you), as well as to officers and some other veterans.
As for vehicles/walkers, Contemptors would be a great option.

Depending on what loadout you give them (you could magnetise the parts if you want to be able to swap them), you could easily use them both as Contemptors as per the FW rules, and as stand-ins for regular Dreadnoughts. As for tanks, I'm not sure what super-heavies I'd go for, but apart from its increased size, the Spartan could both work with the FW rules and as a regular LR stand-in for those non-Forge World games that you might be playing (the Proteus could also work, but it lacks the frontal assault ramp). Maybe use a Baneblade as a counts-as Fellblade (its predecessor, IIRC).
Oh, and for Terminator armour? Yes, the Indomitus is the current pattern that we have a plastic kit for (it says so in Codex: Space Wolves, if you're wondering), and that could work fine (all Terminator marks that we know of were technically around during the late Crusade and Heresy), but since it was likely only limited to Legions like the Imperial Fists (I've always assumed it was limited to Mars and a few other key Forge Worlds at the time, while the Cataphractii pattern was the most widespread due to it being cheaper, easy to maintain, and ultimately expendable), I'd go for either Tartaros with the aquilas/Crux Terminatus icons removed, or convert/wait for Cataphractii.
Assuming you don't limit yourself to the Mark V and VI pieces from those kits, all I have to say is this:
Post-Heresy, Post-Heresy everywhere.