Which overkill shall i get next?

Of the ones you list, you should Wrecking Everything Live, it's very good. It's not as long as Wrecking Your Neck Live (for the WFO tour), but I think the song collection is solid. I'm a big fan of latter era Overkill, and it gets some good treatment. Plus old school fans of the band will like the treatment The Years of Decay gets on the live record and accompanying DVD. Which I HIGHLY recommend.
 
Be informed that I'm not familiar with the Over Kill releases after Horrorscope, 'cause I just like their old stuff. But I'd definitely recommend to pick up Feel The Fire. It's awesome!