Buy used miniatures online and look up methods to strip the paint off. Many of them can revert the models back into just-assembled condition. GW's models are appallingly expensive, and there's always a fuckton of preowned Chaos models floating around online, since they're a fairly popular faction. Trying to find Sisters of Battle or Grey Knights is a different thing.
I need to know beginner stuff for Chaos. I would like to run the Black Legion faction but have read some things that discourage them in small games because they are a specialty army. How small or large do games get?
Black Legion are actually in the backstory the go-to stereotypical legion that includes everything you can find in the codex. Other sub-factions like World Eaters, Word Bearers or Thousand Sons are the actual specialty armies that players often restrict their choices with voluntarily to make an army that fits with the lore. If you thought the Black Legion supplement codex meant they're a specialty army, you got played by GW. The supplement codices are little more than a cashgrab, and you can make a perfectly complete army with just the ordinary codex.
For a beginner I'd suggest starting with basic infantry and expanding from there. I don't know how familiar with the rules you are yet, but they're quite complex and take some time to sink in. Games are rarely larger than 2,000 points, and even those can take like 8 hours. For games beyond that size there's a whole expansion called Apocalypse, which I will recommend any sane person to stay far away from. Try different tactics and setups and find a play style that you enjoy. The good thing about Chaos is their versatility: they can support basically any play style. Like charging head on into melee? Khorne Berzerkers, Maulerfiends and Possessed for you then. Want to blast the enemy from afar? Defilers, Noise Marines and Havocs are your pick. Want to drown the enemy in hordes? Get Typhus, and swamp your enemy with Plague Zombies and Cultists. Want to rush the enemy fast? Heldrakes, Bikers and Rhino transports should get the job done. Like tanking and heavy armour? Land Raiders, Obliterators, Terminators and Plague Marines a go-go!
The Black Legion color scheme is good for beginners as it's simple yet effective, and you can paint a lot of models in a short time. However, it's common practice to put a bit more effort into your commander and elite units, so try to think of other colors that might fit with the scheme to make them stand out more. Black-dominant color schemes can easily end up looking monotonous and unfinished, so get your hands on a drybrush, some washes, and don't be afraid to use a splash of color on details like hair, banners or vehicles.
Conversions with Chaos are very easy, and can save you an assload of money. Instead of buying the Plague Marine models you can just spurt some glue on regular marines, paint them a filthy green and voilá, Plague Marines. You can make Chaos Spawn by just gluing a bunch of spare bits together and it's still 100% consistent with the lore.
Oh, and never ever ever ever ever ever ever ever start playing regular Space Marines. If you do, I will personally transform into bits of data, find you on the internet, jump out of your computer screen and slap you. I've seen enough Space Marine armies to last me three lifetimes over, and so have most other long-time hobbyists.
There's my 2 cents, I'll be happy to hear about how your army is coming together. Good luck on your foray into the world of 40k!