1250 pt chaos?

You can definitely get 1250 out of that. Just using the straight codex, I'd do something to this effect for 1250. It leaves you with some points to play with. You've easily got 2000 points' worth of stuff, though.

HQ/
-Chaos Lord with the mark of Chaos Undivided, combi-bolter, power weapon, terminator armour, spiky bits, daemonic mutation, resilience, strength. The lord has a Chosen Bodyguard.

Elites/
-Chosen Bodyguard, 5-strong. All in Terminator Armour. One Aspiring Champion, armed with dual lightning claws, spiky bits. One Reaper Autocannon. Two power fist/combi-bolters, one power weapon/combi-bolter. The power weapon/combi-bolter-armed model has an Icon of Chaos Undivided. The squad has the Mark of Chaos Undivided.

-Two Obliterators

Troops/
-12 Chaos Space Marines armed with bolt pistols and close combat weapons. The Aspiring Champion is armed with a power fist and bolt pistol, and has spiky bits and a teleport homer. One model is armed with a meltagun, and another has a plasma pistol and close combat weapon. The squad has the Mark of Chaos Undivided and can Infiltrate.

-10 Chaos Space Marines armed with boltguns and close combat weapons. The Aspiring Champion is armed with a power fist and boltgun, and has a teleport homer. One model is armed with a meltagun, and one model is armed with a plasma rifle. The squad has the Mark of Chaos Undivided and can Infiltrate.

Fast Attack/
No selections

Heavy Support/
-Defiler

As for lascannons on the defiler... as it currently stands, you bring a defiler for its battle cannon, not for its other weapons. A dreadnought can fulfill both the close combat role better with its WS4, and the anti-tank fire support role for fewer points, and has a significantly smaller "footprint," if you will.

I've never been a fan of loading the defiler with much of anything. The stock unit comes with everything it needs, really... daemonic possession and a battle cannon.

As for being light on anti-tank... meltaguns. Each squad has one. Coupled with Infiltrators, you'll be able to put them where you need them. A lot of people argue that you should pair special weapons rather than taking two different ones in a squad, but having a plasma rifle and a meltagun in the bolter-armed squad gives them tactical versatility without inhibiting their movement, while the meltagun/plasma pistol combination in the assault squad gives them similar flexibility at range, albeit not as good as the bolter-armed squad against light infantry, and does not impede their charge, nor significantly detract from their attacks in close combat. Two attacks lost from not being double armed by taking a pair of meltaguns, for example, is one double-armed Chaos Space Marine's attacks. Only losing one attack to take a single meltagun and have the other special weapons trooper still be double-armed by having a plasma pistol is a worthwhile exchange, though.

More importantly, though, you're not going to have a lot of heavy armor to deal with most of the time in 1250 points. Even at "standard" 1500 (although admittedly, 2000 has always been the standard for me...) you're only going to see one or two real tanks at the most, and loading up on anti-armor equipment when you're better served by having enough anti-infantry fire doesn't do much for you. Besides, the tanks don't mean a damned thing if the infantry they're there to support are dead. Having a Predator Annihilator, for example, isn't going to help you against a mostly infantry army. Yes, lascannons are good, but when you've shot their one or two vehicles down, then what? It's better to have units that can do something useful the entire game. I'm a huge proponent of tactical flexibility if only for that reason-- if a unit can stay valuable in the fight for six turns in any capacity, they were worth having. If they've done their job in two turns and can't contribute further in a meaningful way, they're less valuable than if they can do their job in two turns and then still provide solid support of a useful type. And really, that's going to be true regardless of what army you're playing... that's not limited to playing an army in power armour, although it's certainly more important since you've got fewer models on the table.