Here's the thing. I bought the Horus Heresy: Age of Darkness boxed set because it has a bunch of Mk VI (beaky) space marines in it. Originally, I'd intended only to use those models with 10th Edition rules, but the Age of Darkness box comes with a 400 page rulebook in it, and I'm nothing if not a sucker for game rules. I read the rulebook and immediately wanted to play Horus Heresy, which frankly feels on brand anyway, given that my first meaningful engagement with Warhammer were the Horus Heresy novels. For a while, space marine legion against traitor marine legion was sufficient, but eventually I started looking at my other 40k miniatures and wonder how they would hold up on a 30k battlefield. Around the same time, I coincidentally purchased some 7th edition codexes, and suddenly recalled that the Horus Heresy game was actually just a revised version of 7th edition rules.
You see where I'm going with this.
With allowance for minor details, model profiles for the 7th edition of Warhammer 40,000 are very usable in Horus Heresy. For the cost of some second-hand books that nobody wants any more anyway, most of my existing models suddenly have valid Horus Heresy stats.
There are minor differences. I think there's a keyword in 7th that doesn't appear in Horus Heresy, and 7th assumes that all models move 6 inches unless some other value is explicitly provided. For the most part though, if it worked in 7th it largely works well enough for a casual Horus Heresy game.
Example tech-priest comparison
Here's an example comparison of a 40k model's profile, and something similar in the 31st millennium. A Tech-Priest Dominus in 40k 7th edition:
And here's a Magos Dominus in Liber Mechanicum:
There are a few minor differences, but they're easy to overlook, accounting for the fact that these aren't literally the same model anyway. A 40k Tech-Priest Dominus in Horus Heresy feels more like a space marine sergeant or praetor than a tech-priest, but then again it's a single model unit. In the tech-priest's wake there are a bunch of Fulgurite Priests with 3 Toughness and 1 Wound each. It'll balance out.
Example dreadnought comparison
There are some models that span, in the lore, both the 30,000 to 40,000 settings. These are especially useful as reality checks, because while you can verify how reasonable a 40k model profile is in the Horus Heresy setting just by attribute keys and values, you can verify it when you see the exact same profile for the exact same model. It also reveals some of the minor adjustments that might be required when porting something from 7th edition to Horus Heresy.
Here are the stats for a Contemptor Dreadnought in a 7th edition codex:
And here they are for a Contemptor Dreadnought in Horus Heresy:
The key stats, WS and BS are identical, as is Initiative (I). The Attacks (A) are close enough to ignore.
However, a Dreadnought was considered a Vehicle in 7th edition but in Horus Heresy it's just classified as, well, a Dreadnought. Vehicles have hull points plus front, side, and rear armour stats instead of a Toughness value, so that just doesn't translate. I looked through some Elite unit profiles though, and it's not uncommon for Strength and Toughness to just be equal on big hefty machines of war (which is indeed the case with the Contemptor Dreadnought for Horus Heresy). That seems like a safe place to start, and after a few games I'd adjust as needed.
Obviously this is just an example, because Contemptor Dreadnoughts have stats in Horus Heresy, but the point is that minor adjustments aren't hard to make when most of the work has already been done..
Why not just make up your own stats?
Obviously Horus Heresy is just a game, and in the end it boils down to numbers. Weapons you have a high Ballistic Skill (BS) or Weapon Skill (WS) in tend to hit more often than weapons you are not skilled with. Projectiles with high Strength (S) or Armour Penetration (AP) tend to break through your enemy's Toughness (T). The more dice you can roll for your Attack (A), the better your chances of success. And so on.
You can make up your own numbers, and the game still plays. I can confirm this. I've done it myself, and I'll do it again as needed, or just for fun.
It's not that you can't reverse engineer the logic enough to understand, more or less, what you're designing. And I'm not under any illusions about game design. I don't believe the game is magically "balanced" because the numbers are printed on paper and that it's magically "unbalanced" when I scribble them on a napkin.
I understand the numbers, but sometimes it's nice to have the numbers already written down for you.
Why not just play proxies?
It's also true that some models basically already have stats, or they nearly have stats. For instance, I own Liber Mechanicum for Horus Heresy, and it's got a bunch of stats for models from the year 30,000, and generally I can see which 40k model I could proxy in as a 30k model. It's not exactly a no-brainer, but it's not that hard, either. You look at the composition of a 30k unit, then you choose a unit from 40k with the same composition. Done!
I could also just buy the 30k models. They look great and they'd probably be fun to paint, but I have to admit that, if nothing else, I just don't have the storage space for a literal endless influx of models.
Those are valid options, but the 7th edition codexes are really good reads (much better than the 10th edition ones, as it turns out) and it removes the need for mental translation. What you see is what you get. The models in the pictures look like the models on your table, and you use the numbers provided. It's too easy.
Playing games
At the end of the day, much of tabletop gaming is about how you translate your imagination into a diorama you get to interact with. You can use tokens on grid paper with illustrations. Or meeples on a board. Or little toy soldiers in amongst model buildings and terrain. In a way, you can never have too little or too much, because neither will ever be as vivid as what's happening in your head. It's fun to bring stories to life with models, and it's fun to experience the surprises that new numbers bring to the rules. If you love seeing how rules interact with one another, try some 7th edition stats in Horus Heresy and see what happens.