@danlegg and @JimHowell1970 thank you for your replies.
I'd preferably buy from companies like Thomann because I'm familiar with 'm (in the sense that I've bought from 'm before).
I didn't know that line level is considerably lower than modular level, I was afraid that I'd "blow up" the modules with output from a synth. Good to know that I sooner need Gain instead of an attenuator.
One of the filters I considered is, indeed, the Doepfer A-120. I saw reviews on Youtube about the Behringer 904A and they said that is was a reasonable replica of the orig. 904A filter (24 dB/oct) from good ol' Moog. If I'm not mistaken then a MiniMoog has sort of the same filter but it appears to, partly, thank it's wonderful sound due to a slightly overdriven input from the oscillator. So maybe I'll have to go for the Doepfer: a brand well respected in the Eurorack community if I'm not mistaken.
I'll do some research on buying second hand in Holland.
Unfortunatly all physical music stores in the part of Holand where I lve have closed. And when they were still open I wasn't impressed by the people working there in the end. So listening to the mudule in question is almost out of the question.
I'll do some more research w/ the help of the extra info I've got now! Thanks.