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So my local meta has its fair share of demon kin. Blood angels. Tyranids. And harliquin. To go with more gun line armies. Being a casual guard player I'm thinking of adding some grey knights for fluff and support. I run a few tanks. Lots of boots on ground. Gun lines mostly.

What units should I buy and equip to help my smooshy human army? Terminator? Purge? Dreadknight? Not looking to create another big army. Just an allied army to the larger guard army.

Dreadknights. Get two.
The beauty of the GK book is the Nemesis Strike Force detachment. 1HQ, 1Troop required. That's it. The troops can be power armored (strike squads) or Terminators. A common force for a small smashy-face ally might be a Librarian (hands down the best/only HQ choice in the book) hoping to get Invisibility or Gate of Infinity/Vortex of Doom to buff 5-10 termies.
You can also get a pair of DKs, which will have the same 2+/5++ (or 4++ with built-in Sanctuary) as your terminator squad, plus they carry some heavy-duty guns.
Some posters will lament the lack of any other option in the codex, which is legitimate. But for an allied force, what else do you need? Sure, it will look exactly like any other GK allied list, but then again that makes it easy to shop for your support guys. I'm talking about less than 10 models, which should be easy enough to sell or trade away if you decide you don't like it.

ld also look into a sqad of purifiers. They add 4 warp charges to your army and they come with a 9" nova that is str5 ap 4 that can also hit flyers. they are possibly the best choice besides NDK's in the codex. Delivery can be an issue, but a rhino or Storm raven fixes that problem fairly well. or in your case a valkyrie

1 Libby, Level 3, Curiass of Sacrifice

5 Termies, 1 hammer, 1 Psycannon

0-2 Dreadknights, Teleporter, Psycannon

Self-contained Nemesis Strike Force, add it to any IoM force.