I'm building my first Eurorack system as well, after some expience with semi-modular synths and VCV rack etc.
I was personally looking at the Fully Wired Electronics Transistor Ladder Filter https://www.modulargrid.net/e/fully-wired-electronics-transistor-ladder-filter. I could be making the completely wrong choice but I quite like the sound of it, with it being a ladder filter and all. Plus (Again I could be completely wrong) but it doesnt seem overly expenisive if I don't like it in the end. From what I've seen I assume it self-oscillates.