As above. An infantry squad is about 50 points, I think and then another 10-ish points to give them a special weapon, like a Plasmagun or Melta. I find the upgrade to always be worthwhile. Basically, a 60 point plasmagun with 10 wounds, immune to multi-wound weapons.
If you want, add another 15 - 20 points for a Heavy Weapon, but if you're on the move, probably not worth it.
Add some officers for the critical Move-Move-Move order, which essentially gives them a second round of movement (including a run move). Getting to the objectives on your first turn, if possible, is money.
Assuming Guardsmen get the updated Power Sword profile, with +1 S, if you take Catachan doctrine you get an additional +1 S so swinging with S5 on your Sarge can be a bit of *something* if you get into melee. It also give them +1
Ld if near an officer, or something like that. Same deal with your Officers. Not great, but if only one or two Marines make it to Close combat you have a slim-to-none chance of beating them in melee, without a Knight getting involved. Power swords are super easy to convert from a chainsword. Leave the "power pack" at the bottom, and then just shave the chain edge to a flat edge with a knife. If you're feeling fancy, you can make a cavalry sabre by shaving the back into the appropriate shape, too.
One other suggestion, might be Artillery that can fire from outside of
LOS. Knights are generally mobile enough to gain
LOS on something, but by contrast you might not want to expose them. Enter the Basilisk, and the Manticore. I'm particularly fond of the Manticore with the Catachan Doctrine, because you get
2d6 shots... and with Catachan, you can reroll either or both of those
2d6 number of shots. So if you get a 6 and a 1, reroll just the 1. Or if you roll a pair of 2's, reroll both. I find it's quite effective at cracking the targets you wouldn't want to put your knights in front of.
But I'd say the biggest thing would be lots of bodies to hold objectives.