I recorded for a while now with Garage Band. Since I want to improve and develop my recording skills I am planing to switch to Logic pro to learn to record more professionally. Well as you might recognize I am a Mac user. My old mac (2011) is pretty slow and used so i plan to buy a new one. And that's exactly where my question's about:

Which Mac Book pro would you reccomend for recording with Logic?

I'm not sure if a i7 processor is necessary. My current idea is an "Apple MacBook Pro Retina (15.40", Intel Core i7, 16GB, SSD)". But I'm not sure if I could also work without any problems with an i5 processor...

The i5 should be fine as well, it might take a performance hit if you're planning on using too many virtual instruments but you should be OK. I have a friend that uses the Mac Mini (i3) processor and runs Logic with no issues at about 24-40 tracks with some guitar vsts. Not a heavy production per se, but it works so you should be fine.
Don't get me wrong - the more you spend, the better, more RAM will probably be the first and most important, then more drive space or a second drive for the music. For me the biggest crutch would be the 15in display - I work on a laptop now and have a 25 in side monitor and whenever I do on location recording it gets really annoying to have to work on a 15in screen.
If you can do a mixture of audio lighting interface with USB external drive so that way you don't tax the system drive which would be mainly running your OS and Logic. The idea here is that one device takes the USB bus, the other the lighting port, so you have the best data throughput. If you already have a USB audio interface, then consider a second drive in lighting or whatever other options you might have (firewire?).