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So you can grab a heavy weapon that has good aim shot accuracy (say. In power armor, you're back at 50 accuracy. So you get a recruit with ~55 accuracy, train him to ~65 accuracy, now he's worthy of power armor.
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Space Marines veterans have to train in order to be granted terminator honours. You have to be an amazing warrior to be able to fight in terminator armor. Terminator armor (if you ever make it) decreases melee by 40 and firing by 30. That represents the fact that new recruits can't immediately be put in power armour, they need to train first otherwise they'd be useless. BUT Power armor also decreases your firing by 20 and melee accuracy by 10. It also increases strength a decent amount (So you can carry lascannons, power fists, plasma cannons, etc, which are really heavy so that scouts can't use them), increase reactions a bit (combat feed in the helmet) and stamina a LOT. Power armor is the equivalent of XCom power suits: It is really strong armor. As firing accuracy becomes better, bolters start making more sense (150% to hit is no better than 100% to hit, after all, and is less interesting than a 4-shots autoshot at 50% to hit, that explodes on impact) Not super glorious but you're just a neophyte, that's ok. Sniper rifles are great weapons to kill Xenos while standing behind a screen of PDF grunts. Scout armor is the equivalent of XCom Personal Armor: It helps you survive but by no means makes you invincible. Maybe some crafts and tanks too, but those are difficult to implement properly. You get your recruits in the shop, as usual, for whichever currency you decide to use (I'm thinking "Astartes Influence", space marines don't use money), you can also buy basic weapons (bolt pistol, chainsword, combat blades, bolters, flamers, heavy bolter, missile launcher) and their ammo there. That explains why you start with soldiers in scout armor. You yourself are the castellan of the keep and will not take part in fights, your duty is to defend the keep.
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The story starts on a world with a chapter keep, doing recruitment: You only have recruits as your soldiers (since that's how the game is). That makes their life suitably cheap and meaningless but that also makes them good fodder in the early game to preserve your scouts (so they can improve and become real space marines). PDF are the expendable grunts (and they can't become Space Marines, they are grown men, not young candidates) -> PDF soldiers are HWPs you can buy for cheap. The main interest is space marines: Make the actual soldiers space marines, no IG (Imperial Guard) or PDF (Planetary Defence Force) I've been spending a fair bit of time day dreaming about an XCom:40k and rereading your thread, saw that you were open to ruleset suggestions.
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