Wich champ should i buy?

LeBlanc and Kha'zix

Rengar is definitely more mechanically intensive when you use him as a full assassin since you need to pull off perfect combos within the time it takes to start your leap from your ult and land on top of your enemy. if you don't get them within about 1-2 seconds of your leap, you are as good as dead. He is a true "high risk/high reward" assassin in that sense.

Zed is only challenging when you want to pull of really crazy pro level plays. Actually assassinating someone is easy on him, all you need to do is R them, drop a shadow when you arrive, Q+E and then AA>BotRK active>AA, and then R to get back out.

Basically, Rengar has a very high skill floor but only a moderately higher skill cap (he's difficult to learn and use effectively, but once you get your combos down, he isn't incredibly hard to use to his full potential) Zed has a low skill floor, but a very high skill cap (he is easy to use effectively, but if you get really good, you can pull off some insane stuff, and it may take you a very long time to master him).

We need more information than that. Like your playstyle, which lane you prefer and how often will you be jungle or mid, more knowledge based champs or more skill based, what type of champions are you looking for, etc etc.

Zed's combo is extremely situational... There's no set combo. He isn't a simple champion. If you want to put a lot of time into learning him, you'll do well with him. If you just want elo, play somebody like Yi.

That's all the ones I know of who can go mid and jungle and a few can even go top lane. I just don't know if your style is sustain, poke or kill. So I'll give a run down on the champions primary and secondary styles. Poke/kill Kill/sustain Poke/kill Sustain/kill Sustain/poke Sustain/kill Kill/sustain Poke/sustain Poke/sustain Sustain/kill Kill/poke

Generally sustain laners counter poke laners, poke laners counter kill laners and kill laners counter sustain laners. There are exceptions to these rules though.