The basic Allied Detachment for Grey Knights is really simple;
Librarian; ML3, Hammer, Book - 165 Points*
Terminators (x5); Incinerator, x1 Hammer - 185 Points
Nemesis Dreadknight; Teleporter, Heavy Incinerator, Greatsword - 190 Points
There's not a lot else you really need out of the GK book, and it's 540 Points. You could throw in some Psycannons, but I prefer Incinerators simply because they're S6, AP4 Ignores Cover, and the Heavy one is Torrent. If you're going to be up against Tau, AP4 is going to wreck them, unless they're all 'Suits, all the time. In which case just Melee them, and walk through their Overwatch in your 2+ Armour. I'd prefer a ****-ton of Hammers (buy an Assault Terminators box, use the Hammers, and then start building towards a Blood Angel Lightning Claw Deep Strike-and-Charge Detachment), but since you're barely starting, I'm only including one Hammer in the unit because there's only one in the box.
To take it to a CAD, you basically just do that same thing, again.
Librarian; ML3, Melta Bombs, The Soul Glaive - 160 Points*
Terminators (x5); Incinerator, x1 Hammer - 185 Points
Nemesis Dreadknight; Teleporter, Heavy Incinerator, Greatsword - 190 Points
* Remember that GK Librarians actually can fight in Melee. You don't have to roll on Sanctic all the time. Sometimes it really is worth it to roll for Electrodisplacement, especially since your Dreadknights can move 30" on Turn 1 and be prime targets to switch with, guaranteeing you a first turn Charge with Terminators.
inb4; "Grey Knights is such a bad Codex, it's all a mono-build." Sure. Okay. Whatever.
At this point you're at 1075, with +10 Warp Dice. Going full douche mode, you can drop the Soul Glaive Librarian for Celestine.
Saint Celestine - 200 Points
Interceptors (x10); x2 Incinerators, x2 Hammers - 280 Points
-160 Points
+480 Points
1395 Points
There's your <1500 Point Detachment. Throw in an Inquisitor and/or an Astropath and/or a Priest or two.
One of the main (i.e; Best) GK/Guard builds is the following;It doesn't need to be a full army, just something that can support a mixed Imperial Guard army (ie. it has both infantry and support tanks/artillery, but only 1 unit of Ogryns for melee). Chances are they'll mostly be up against Tau, not other Marines or Eldar.
Brother-Captain Stern
Ministorum Priest; Litanies of Faith (Imperial Agents!!!)
Bullgryns; Power Mauls and Brute Shields
Brother-Captain Stern always comes with Sanctuary (it's the only reason he's viable). The Bullgryns have the Brute Shields that give them an Invulnerable, which is increased by Sanctuary, and when they get to Melee, they can re-roll their Invulnerable thanks to the Priest...The Priest also brings Zealot, which does matter. An important note is that Imperial Agent Priests can be taken in a Detachment by themselves, and bring a Relic that makes them automatically pass their War Hymn roll, which is pretty handy on a model that's only Leadership 7.