These should be your three next purchases. With those you can easily get it to be 1000 points or so:
++ Battalion Detachment +5CP (Chaos - Thousand Sons) [56 PL, 1054pts] ++
+ HQ +
Ahriman [7 PL, 131pts]
Daemon Prince of Tzeentch [8 PL, 156pts]: Hellforged sword
+ Troops +
Chaos Cultists [6 PL, 80pts]: 19x Chaos Cultist w/ autopistol and brutal assault weapon
. Cultist Champion: Brutal assault weapon and Autopistol
Rubric Marines [7 PL, 107pts]
. Aspiring Sorcerer: Force stave, Inferno Bolt Pistol
. 4x Rubric Marine w/ Inferno Boltgun: 4x Inferno boltgun
Tzaangors [7 PL, 150pts]: Brayhorn, 19x Tzaangor w/ Tzaangor Blades
. Twistbray: Tzaangor blades
+ Elites +
Scarab Occult Terminators [11 PL, 238pts]: Hellfyre Missile Rack
. Scarab Occult Sorcerer: Force stave, Inferno Combi-bolter
. 3x Terminator: 3x Inferno Combi-bolter, 3x Powersword
. Terminator w/ Heavy Weapon: Power sword, Soulreaper cannon
Tzaangor Shaman [5 PL, 90pts]: Force stave
+ Fast Attack +
Tzaangor Enlightened [5 PL, 102pts]: Aviarch, 5x Enlightened, Fatecaster greatbows
++ Total: [56 PL, 1054pts] ++
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I'm not saying that this is the best build but it's not the worst, and your playstyle defines a lot. I personally like cultists. I know others don't. After this, flesh it out based on how you feel comfortable playing, but a Getting STarted box of Tzeentch Daemons, a Mutalith Vortex Beast, or Magnus the Red aren't terrible choices in any case. Nor would another Arcanite Changecult be.
I have two lists I'll share with you - the 1KSons/Daemons list I use, and the 1KSons/Daemons/Renegades List. I find the latter most flexible but I also prefer shooty.