So if I understand you correctly you're one troop short of a Battalion.
If you want something different from Rubrics, there's two choices here: 10 cheap Cultists. Or a BLOB of cultists (Thousand Sons don't have the same restriction on Cultists that CSM do. However, you can use their strat in a game when you use your CSM as part of an army to bring Thousand Sons Cultists back from the dead. Cheeky, I know!)
That's the cheap option. The option I currently use is Tzaangors. I never get mileage out of them but that's me. A lot of people love them. I only use about 18-20 and deep strike them nearly every game to harass my opponent's gunlines. It also forces them to deploy differently.
Moving on to your shooty problem.
Since I'm sure you'll learn that short of getting a Knight, or Forgeworld, our Psychic phase is where our best vehicle damage comes from.
So in no particular order the units that 'finish' off what the Psychic phase started are:
- Helbrute: Las/missile (very cheap)
- Decimator: Butcher Cannons (semi-cheap) (I have Soul Burners on mine but I ordered it literally the day they went up a ton of points.)
- Contemptor with Two twin lascannons. (I have been using this more, but it's a bullet magnet; even though it is a victim of RNG quite often, the top range of the damage curve is quite high.
- Skorpius: (Note that's 3 units in this list already from Forgeworld which is telling you something). This is expensive to put on the table but it does fill a massive void for Thousand Sons. Indirect fire is hard to come by for Chaos, but Marines are so good, it's almost nice just to have this as an auto Primaris killer, and potentially take care of some Thunderfire Cannons.
- Leviathan. The Chaos one, I've found, is not as good as the Loyalist one (shocker!) But it's still a beast and hard to take down.
- Deredeo. If you can get past the look of this science fiction washing machine, it is exceptionally potent. Expensive, but it will be a priority for your opponent...especially with all the fly in the game right now.
In the area of, "If you can get these cheap:"
- Predators. We still have that Strategem that gets you an advantage with three of them. But I just have had very, very little success with this unit.
- Vindicators. Although I've been singing their praises since the last point reduction, I still don't own 3 of them, but 2 is good (three gets you the mortal wound shell) They are cheaper, D6 shots now, and T8.
- More Helbrutes: It's not too hard to convert these since that old plastic one from the box set is all over ebay. I like Helbrutes more now because we can use Duplicity to slingshot them across the board. I think even the vanilla Multi melta/Fist is a cheap scary unit for your opponent.
I think the 'Daemonic Engines' are all very bad. That was how GW planned to sell the Disco lord since he automagically makes them mediocre. Still we don't get him, so this makes (in my personal opinion) Daemon Engines a less than stellar choice for what you want to use them for.
I gotta go, but that's a fair amount to chew on plus the other great advice you've been given. Good luck, and welcome!