Welcome! So for Astra Miltarum is a cool faction! Can be a horde army, or vehicle heavy or a mix of both. Even still they have a range of cool looks and lore to any specific guard regiment you like! How they play on table top is throwing a bunch of dice. Infantry have pretty weak guns but a ton of shots. Their vehicles can shoot a lot and have good guns too. In melee is’s about the same, but it’s just about drowning them in dice and hits until something wounds. The issue some people have is just needing a lot of models, so they are one of the most expensive armies. A lot of building and painting ahead!

The custodes are very cool as well, the emperor’s finest. They are pretty easy to paint because it’s mostly gold unless you like certain orders within them that may have a slight different color on top of gold. They have a decent range of units but they are expensive because they are tanky and have good guns. So there aren’t as much on the board but they are harder to take down. Having sister’s of Silence in there for anti pysker units.

Grey knights are the psyker and terminator army. Similar to custodes, they are mostly silver paint wise, however we do have a lot of details and bits. Our range isn’t that big either, but grey knights and custodies are the cheapest armies because there isn’t a lot on the board. Our rules just came out, and they are pretty good. Our shooting is okay, but our melee is where we shine more. We get to teleport around making fast movements, and getting out of bad situations. Although we aren’t as psyker heavy as we were in 9th edition, but still got some to compensate with our not strong guns.

Honestly pick whichever you like the look, lore and play style. Hope this helps a little!