I got a 50l air tank from my workmate that he no longer needed. 

It's a nice coincidence, that it fits perfectly inside the shelf with the fridge compressors.

Fillingling the tank up to 6 bar takes around 2:15min, which I think is very good for plasma cutting, pneumatics, and spray painting.

So, I'm pretty happy with the new tank and the only thing that's left to do is adding another oil separator some tubing, and a muffler to make the venting more silent and the floor less oily.