With Version 3.171 I finally arriving I was able to trigger the "Avenger" for my first time. (Thanks Keith)
Basically the avenger is a huge fortress with 200.000.000 health.
What makes the Avenger even more challenging are the 13 modules (in my case), which give it some extra stamina and firepower.
I included a screenshot which gives the modules I encountered. (I dont know if the numbers and kinds of modules differ according to the difficulty level - no information on the Wiki)
So anyway, with speed 5 it took the station some time to reach my chokepoint 4 hops away.
I included a screen shot of the warp point defence - I wasnt sure how hard the battle would be so I bought everything I could until my energy ran out.
OK, Keith - to your question if/how I took down the Avenger
The avenger lasted for exactly 50 seconds until it disappeared into thin air
taking 99% of all my ships and nearly all of my installations with it
Nice job of designing this fortress - but maybe you could scale the health a little bit up for higher difficulties