I thought I'd go through the plastic box sets available at this time and list a few bits from each that are useful to the Pre-Heresy modeller.
First though, the big ones to avoid:
Unless you are doing Emperor's Children, this torso should be avoided due to the Aquila. If you have no other suitable torsos, you can always file this one down - the abdominal cables aren't truly indicative of any specific mark.
These are all Mark 7 helmets. There are other variations such as the one in the Commander box set as well as quite a few in the Chaos range as well. The identifying feature is the mouth grill. If you smooth down/putty over that, you can make the head usable.
SM Tactical Squad
These two torsos have a winged skull emblem on them. They're perfectly acceptable pre-heresy (though the one on the left has cable protection that was only introduced in MkVIII Errant armour), but do even better theme-wise for a Siege of Terra/Scouring-period armies as the winged skull was the loyalist symbol used.
This one is, again, perfectly usable for pre-heresy and very useful for boltgun-armed marines. I personally like to remove the skull symbol.
This is just about the closest you'll get to a plastic MkV that doesn't need conversion.
Theoretically, this is supposed to be a MkVIII chest, but they forgot the abdominal shielding for the cables when they sculpted it, so it's really a Mk.VII with MkVIII-style collar added. I tend to use this one for the collar alone, trim off the wings, vox and skulls and sculpt any details needed.
These two heads can be used for MkV conversions when you add studs to the skull cap.
A very popular piece for those looking to convert MkV and MkVI armours.
Assault Squad
The only piece of specific use to pre-heresy modelling in this box is the chestplate. This chest can be converted to either MkIV or MkV depending on what you replace the details with.
Command Squad
The apothecary torso is another that would be perfect for Siege of Terra or Scouring. If you'd like it to be more clearly earlier in the timeline, just file it down, or take the belt-mounted vials and pouches and stick them to another, more appropriate torso.
Though I don't like the wooden plank of a tabard on this chest, the armour itself is a great shape. As with the errant chest from the Tac Squad, I would tend to remove the wings, purity seals and skull and sculpt more appropriate details.
The champion's head is very good for use as a MkIII helmet.
I've seen the champion's shoulderpads used, along with standard pads, to bulk out the legs on terminators.
SM Commander Box
Another winged skull chesplate. Would make a good Scouring commander or standard bearer chest.
This piece is perfect for use on Cataphractii terminators.
Vehicles
Dreadnought shin plates are probably the most commonly seen bits when it comes to Cataphractii shoulder plates.
The first one is from the Land Raider Crusader and the second is in the generic vehicle accessory sprue. Both would be suitable for Techmarines (though a little plain) or Iron Hands and, as with the skulled one in the tac squad, can easily be converted.
As mentioned in the previous article, this shoulderpad is perfect for Iron Hands clan symbols.
Black Templars Upgrade Kit
This is the only plastic example of a MkIV chest. Hopefully, you either collect Black Templars or have a friend who does and is willing to part with them.
I would use this one, again, mostly for the collar.
All the Black Templar torsos could be used for the Imperial Fists Legion's First Company. Variously called Huscarls or Templars.
The three suitable heads that come in this set.
Dark Angels Company Veterans
These two torsos could literally be used for anything, ranging from a Word Bearers marine to a Thousand Son sorcerer (or of course, a Dark Angel).
These are more likely to be useful to only Dark Angel players, but are still perfectly acceptable as pre-heresy.
Another Siege of Terra/Scouring chest.
A good head for a Librarian of any Legion.
Another studded shoulderpad, though the chapter icon will need covering up if you're making a different Legion.
Ravenwing
Another couple of suitable chests.
A suitable chest if you're doing a Dark Angels army from the Order of the Ravenwing.
Two more helms. The right one would be especially good for Techmarines/Iron Hands.
Old Space Wolf Bits
All four of these would be suitable torsos to be used - with a bit of trimming if you're not a Space Wolf player.
This is the only true MkVI chest in the current plastic range. It's a shame the tooth necklace precludes its use for most other Legions. Would suit Space Wolves (of course), White Scars, World Eaters(?) and maybe even Luna Wolves. You would probably need to cover up the Fenrisian rune for any of the other Legions though.
SW Pack
All of these would work quite well, though the bottom-right one does have a MkVIII gut plate.
Chaos Space Marines Set
All of these would be suitable once skulls have been removed and spiked filed down/squared off. The bottom-centre one may well make a good basis for a MkII chest due to the ridged pipe going down the centre of the chest.
I've seen this head used for MkV before.
Though they all have horns, any of these heads would be suitable once they've had them removed and new earpieces have been converted for them and putty used to go over the more overtly chaos elements. The first four would be suited to MkIII armour. The bottom-centre could work well as the helmet for a recon marine or devastator squad. The last would, of course, be suitable for Thousand Sons, though any trooper could get away with it too.
With a bit of putty for the jaw - and removal/squaring off the arrow, this helmet would be fine. The jaw could be left unmodified for a chaplain.
Bezerkers
Once the Khorne symbols have been easily removed, none of these chests would require any more modification.
One of the most popular heads for pre-heresy modellers as it matches the helmets from some of the book covers perfectly (once you remove the bunny ears).
While it may have a mouth-grille, this helmet matches the World Eaters art in the Collected Visions artbook really well.
A very cool, archaic-looking plasma pistol.
CSM Vehicles
Same as the rest, though you may wish to cover the skull with a vox-grille or somesuch.
Another helmet ripe for MkV usage.
The two heads from the Defiler kit would be good for a pre-heresy dreadnought, though I'm personally not too sure if they work in terms of size. The right-hand one would be especially good for Iron Warriors.
As you may have gathered, while you can go the whole hog with armour mark accuracy, you don't have to. The vast majority of the torsos I've shown you have been MkVIIs, but they're different enough to give the right "feel" to a force. Personally, I would go without the winged skull torsos unless I was doing something in the late Heresy period. While they are canonically fine, they are too reminiscent of the Aquila torsos in my opinion. But then, you can always file them down and change them as you see fit.
Hopefully this will help anyone going for a pre-heresy theme to locate bits that might be useful to them.