I recently got questions for a couple of folks I referred to 
This article focuses on CHAOS LEGION CORE. If people find it interesting, I'll repeat it for other sets & editions.
Note that the "tiers" (A/B/C/D) are not based on the complete meta but rather are used to indicate where the priorities should be for NEW players.
RENT vs BUY
Typically I recommend people use rentals liberally to try out cards and learn the game, however I would be remiss not to encourage ownership, as ultimately I think all serious players should own the cards they play with. For most people this is a gradual process, and there's no shame in buying a few cards initially and complementing that with rented cards.
MARKET LIQUIDITY
There's been increased demand for cards recently (I'm partly to blame, as I've been buying lots of Chaos Legion cards...). As a result, there isn't a ton of liquidity. I.e., you sometimes have cards where only a few copies are available on the market. Let's take a look at Acid Shooter again: right now buying a max copy would cost $155. Please do not buy a $155 Chaos Legion card... Either rent it or wait for liquidity to return to buy it.
REGULAR FOIL OR GOLD FOIL?
All my recommendations below are based on current values of Regular Foil cards. However, you can sometimes get a Gold Foil equivalent for not much more than the Regular Foil, and in that case it's worth considering the Gold Foil. The primary value you'll get from Gold Foil cards is a 10% boost to reward multipliers when you win a battle.
E.g. as I'm writing this I see a Regular Foil Deeplurker going for $7.50 and the Gold Foil version going for $10. Given how good that card is, it's probably worth buying the Gold Foil.
WHY CL? ISN'T IT GOING OUT OF MODERN SOON?
Yes, Chaos Legion is going out of Modern soon. But soon is ~3-4 months, which means you still have ample time to use it in Modern, and it'll still have utility in Wild and Survival afterwards. It's also much more affordable than Rebellion, and also affordable compared to other older Wild sets.
SHOULD I MAX IT? A QUICK LOOK AT POWER CURVES
Each card's power curve is different, and I don't cover that here. You could write a whole book to cover each card's power curve... Here I focus only on max cards (except for fiends, where I recommend 1 BCX only if you're on a budget), but depending on your budget you should look at the power curve / progression and decide how much BCX you really need.
For example let's look at "Goblin Psychic". A lot of people stop at level 7 where he gets Silence, making him a beast in Magic-heavy rulesets. Theoretically he's viable at level 1 with the Tank Heal as well, though he's very squishy with only 3 Health there.
Now let's look at "Acid Shooter". This is a very powerful card in the current Champion-level meta, where "sustain" games have heavy tanks up front that are very hard to kill. Acid Shooter's scattershot gets him to attack the back-line, but it's the Poison at level 6 (max for Epic) that makes him so important.
IF I CAN, SHOULD I BUY EVERYTHING?
No, don't buy everything. Firstly, there are some cards you'll never use (marked as "Do not buy" below). Other cards (C tier mostly) are situational, so only get those if you find yourself with a gap in a certain area. If you start with A/B tiers those should cover most of your needs.
SUMMONERS
Your priority should be to buy summoners for each Splinter. If you're on a tight budget, don't buy multiple summoners from the same Splinter, so for example pick Kelya OR Possibilus, to give yourself more versatility with your deck.
A tier: Buy <$30
- Kelya Frendul
- Lily Shieldpaw
- Immortalis
B tier: Buy <$20
- Possibilus
- Thaddius Brood
- Obsidian
- Conqueror Jacek
- Quix the Devious
- Astral Entity
- Grandmaster Rathe
C tier: Buy <$10
- Tarsa
D tier: Do not buy
- General Sloan
UNITS
After you have a good supply of Summoners that let you play with different Splinters, look for cards within each that are in the top (A/B) tiers. Since there's low liquidity in the market, make sure you're not overpaying (use guidelines below). Rent if you can't buy at a reasonable price. Try to get a good mix of cards in terms of type (melee/ranged/magic), splinter, and mana cost in order to be able to compete in different rulesets.
LEGENDARY UNITS
Fiends: Buy 1 BCX as filler card for <$1, or buy max for $6-8 to get the ability at level 4.
S tier: Buy <$12
- Adelade Brightwing
- Baakjira
- Queen Mycelia
A tier: Buy <$10
- Chaos Dragon
- Spirit Hoarder
B tier: Buy <$8
- Void Dragon
- Djinn Muirat
- Carnage Titan
- River Hellondale
C tier: Buy <$5
- Desert Dragon
- Lira the Dark
- Uriel the Purifier
- Legionnaire Alvar
D tier: Do not buy
- Ifrit Rising
- Zyvax Vuul
- Grum Flamdeblade
- Iza the Fanged
EPIC UNITS
S tier: Buy <$12
- Acid Shooter
A tier: Buy <$9
- Djinn Inferni
- Magi of Chaos
- Wave Brood
- Igor Darkspear
B tier: Buy <$7
- Dax Paragon
- Grund
- Gem Meteor
- Forgotten One
- Nerissa Tridawn
C tier: Buy <$5
- Weirding Warrior
- Prismologist
- Magi Necrosi
- Tusk the Wilde
D tier: Do not buy
- Hunter Jarx
- Temporal Master
- Insidious Warlock
RARE UNITS
S tier: Buy <$12
- Merdaali Guardian
- Xenith Monk
A tier: Buy <$8
- Regal Peryton
- Tenyii Striker
- Time Mage
- Pelacor Arbalest
- Angelic Mandarin
- Diemonshark
- Mycelic Slipspawn
- Cursed Windeku
B tier: Buy <$5
- Soul Strangler
- Celestial Harpy
- Mycelic Infantry
- Xenith Archer
- Venith Spellsmith
- Goblin Tower
- Life Sapper
C tier: Buy <$3
- Djinn Apprentice
- River Nymph
- Scavo Firebolt
- Molten Ash Golem
- Dhampir Stalker
D tier: Do not buy
- Portal Spinner
COMMON UNITS
S tier: Buy <$12
- Goblin Psychic
- Scavo Hireling
- Deeplurker
- Chaos Agent
- Supply Runner
A tier: Buy <$7
- Kulu Swimhunter
- Riftwing
- Stitch Leech
- Antoid Platoon
- Scavo Chemist
B tier: Buy <$5
- Disintegrator
- Silent Sha-vi
- Cruel Sethropod
- Flying Squid
- Hill Giant
- Crypt Beetle
- Blinding Reflector
C tier: Buy <$3
- Chaos Knight
- Lava Spider
- Hardy Stonefish
- Venari Scout
- Venari Knifer
- Carrion Shade
- Mycelic Morphoid
D tier: Do not buy
- Shadow Snitch
- Sunkai Harvester
- Gargoya Scrapper
- Radiated Brute
- Radiated Scorcher
The above tiers are my own personal recommendations based on my play experience. Furthermore, they are obviously NOT financial advice. They are also based on current market values, so depending on when you read this the market may look very different!
If you have any feedback on my ratings, please comment below.