Haven't bought a single key yet, I do have alot of these crates.
The breakout type S and xdevil mk2 look pretty ok I guess.

Yeah, to be honest these crates have some ridiculous value, but they also drop very rarely. Maybe that's the reason they are so expensive and maybe also because we can't buy them from steam market.

If you're interested, 1 crate = 1 key, so if you bother enough to get into trading, getting keys is super easy without spending a penny on the game
-----
Also i wouldn't agree that it is super hard to get crates. I mean it's probably down to many factors like chance, how long your games last and so on, but I've played the game for around 100 hours and I've gotten at leas 30 crate drops

Depends on the crate, and when that crate got introduced. When the crates first came out it was possible to get like 6 keys for 1 crate, but now you can barely get any keys at all for c1 and c2 crates, and even though c3 crates aren't that old the rate of those also already fell below 1:1. So I wouldn't say it's very easy to get a key for a c3 crate