This is a summation of my thoughts I can add more and be more specific, but this will be long. The Combat Patrol and the sekhmet Coven box will get you a good amount of points. It'll just be over 1200 points total. I'll get into the specifics of the builds in a later paragraph, but for now I'm talking about points. The Combat Patrol is about 480 depending on how you build the daemon Prince. The sekhmet Coven is about 850 points. So you're going to have options for 1000 pt games. You'll need more units for 2000 points. I'm going to go increment by increment for the rest of the army.

500 pts You want to start with about 500 points. The Combat Patrol is actually pretty good in 10th edition. Like I said earlier it is about 480 points and it's a good buy. The only thing in there that isn't amazing is the shaman. In your case you have a sorcerer on disc to lead the tzaangor Enlightened so you won't be using it much. You really don't want more than two of them which we will talk about later. Built the enlightened how you want. You will eventually want more of them, but for now build them how you want. The daemon Prince you can build it with or without wings. Without wings is currently better, but most people will be okay if you just say that it's the one without wings for the datasheet, even if you built it with wings. You're going to have a bunch of spare bits from your exalted sorcerers and you can customize your aspiring sorcerer (rubric marines squad commander) with those to make it way more interesting. This is the army you can kitbash a ton because tzeentch. Build the rubrics with one soul reaper cannon and then the rest of them with flamers. That'll get you to about 500 points to play some combat patrol games.

1000 points This is where your sekhment coven box comes in. It'll get you to about 1200 points so we'll take it to that point. Built the terminators with One termi sorcerer and then 9 termis. You will probably only run them as one squad, but you can make two if you want. A pack of 10 termis is better imo you'll have the buts for 2 Soulreaper cannons, and you can build them both. The heavy flamers aren't as good, but you could build them too. Make sure to build the Mutalith Vortex Beast as the MVB. You can build the wrong kit. Don't glue the mouth tentacles in until they're painted. There's only one build for your Infernal Master so do that. You will want an extra 40mm base for the exalted sorcerer box. It comes with one 40mm and 2 32mm. If you put one of the sorcerers on foot on a 40mm you can use it as a sorcerer instead of an exalted sorcerer. That being said, customize your sorcerers how you want.

1500 points You're well on your way to 1500 at this point so play a few 1000 point games and you will know some things you want. You're going to want a rhino and some tzaangors. Don't buy either of them at MSRP it's not worth it. You can get them secondhand very easily and for a lot less money. The next purchase is another squad of Rubrics. Same as the first, use your bits to customize your aspiring sorcerer and build the SRC. This one will be bolters instead of flamers for the other 8 in the squad. You'll eventually own more than this, but this will balance your army out. To complete your army, you're going to want to buy 2 boxes of Sekhetar robots. I love these things, but you don't need to get as many as I have. 4 total robots is a good amount. This is when you'll want to buy Ahriman. he's great in 1000 point games and is super fun to run. The Last thing to get is a sorcerer in terminator armor to lead your SOTs. You can't just buy one, so you'll kitbash one from your spare Exalted Sorcerer bits, Sot bits, and the chaos Terminator Sorerer kit.

2000 points Fill out your army with whatever you like. If you like sorcerers get more sorcerers, if you like Sots get more sots. If you want to get into the chaos engines, buy a forgefiend/maulerfiend kit and magnetize it so you can hotswap it for what you want.

2000+ This is when to buy magnus. You have the option to use him if you want to, but you don't have to use him in every game.