I read from Guilty Gear wiki that you could unlock Kliff and Justice around mid 200 levels in survival mode. After getting to level 380 on beginner and level 240 on normal with Faust (My favourite character in additon to Slayer, May and Potemkin) the secret characters are nowhere to be seen.

I'm assuming that either wikis information is false or some special conditions need to be met. From what I've read, the proper ways to unlock these characters is to have enough time logged into the game (about three days) or complete enough mission mode tasks.

Also, do you get any kind of unlockables and/or other special stuff making far enough in
survival mode? Or is it just for fun? Wouldn't really mind as I love that mode.

Thanks! Justice and Kliff will probably show somewhere around 500 in survival, just my guess. I'll get there eventually and see if it is so.
Being working on mission mode too and I'm about 10 missions short of 50 (which is supposed to unlock secret characters), so that seems to be valid choice to get J and K too.

You can also unlock everything in the game by fighting for I believe 30 hours. Not having the game on, you have to be in a physical fight or in training. I think that's how it works.

Small update regarding this issue:

I managed to get to level 460 on normal with May (EX). Kliff showed up at level 440 and I beat him in a very close fight. Justice showed up at level 460 and absolutely ravaged me with her beam spam overdirve. That move is ludicrous; It takes over 1/3 off your healt even when you are blocking.
I was so excited that I forgot to use the perfect defence (?). If I could have perfect blocked those beams I would have beaten her quite handily.
So the correct survival levels for black Kliff and Justice seem to be 440 and 460. After that I guess that golden characters start to show up.

Oh well, still something to achieve in the future.

As a side note: I was extremely pleased how easily I whacked my nemesis, the black Robo-Ky. Take that you heap o' bolts!

If you think that unlocking black Justice is hard, then you'll cry when/if you'll reach gold versions of characters. They don't have unlimited tension, but deals increased damage and every one of them are full of nasty surprises, for example: Slayer regenerates his hp over time, all his attacks are vampiric (he heals himself by dealing damage to opponent) and he's just insane powerful, Chip is even faster and more powerful, Sol is in perma Rage mode(or whatever it is called) and regains hp, etc... Anyway, good luck trying to unlock everything.

Yeah, I've gotten taste of golden characters in mission mode. They are quite ludicrous.
I think I'll settle in beating the dark Justice for the time being. I've never been hardcore fighting game player and play them mainly for fun. My current all-time favourites would be the One Must Fall 2097, Guilty Gear X2 Reload and Smash Bros. series.
Also, I have way too many other games to play to unlock everything in a fighting game that requires huge amount of training.