After collecting the above requirements, i started to plan on how to achieve these goals:

Portable

It should be small enough, so one person can carry it. Since one destination will be a music festival, also two people carrying it alongside other stuff can be possible. So not wider than 50 cm and with handles on each side.

Long battery life (>15 hrs)

I do not have that much experience in scaling battery size. Therefore i will go with, the more the merrier.
The budget limit would be very problematic, but fortunately i have several big battery packs from a dismantled Formula Student race car. More on that later., but it saved me the big bucks in Li-Po batteries.

Strong housing

Since my father is a hobby-carpenter and my girlfriend a hobby-painter, I outsourced the housing to him and painting the box at the end to her. I will however plan everything in CAD before starting any construction.

High connectivity (USB, Aux cable, Bluetooth), Decent maximum volume, Budget around 150 EUR

These together have puzzled me for some time. I made the conclusion that i good compromise will be a car radio. This way i do not have to build everything from scratch (no experience in audio amplifier construction).

A nice new car radio with two or four speaker should be enough to cover all three requirements.

Additional features could include: USB outlets, lighting for tent/hut, FM radio reception

Throwing in some 5 V converter, a connector for some sort of LED lamp and an antenna for the car radio will not be that hard.

That should be enough to get some parts and start drawing some schematics!