I'd need to:
1) - open mp3/wav files, do some trimming and fade ins/outs and save to
mp3/wav
2) - do some recordings of JACK-apps

Although you mention the ability to load MP3 files as a necessity, I would advise you
to convert them to WAV, import them into Ardour as regions, and then edit to your hearts
content.

Advantages: No damage to audio files it things get messy, Ardour has lots of great features
Disadvantages: A little more work to convert files, and they take up a little more space...

How to convert?: I would use Sox, or SoundConverter.
Sox is a command-line app that can convert anything to everything.. :-)
SoundConverter is a GTK+ sound converter, does the basics of MP3, WAV, OGG, AIFF etc.

Hope that helps! -Harry