I am new to the game and have been playing around only in sandbox.
A simplified ship with 2x railguns, maybe 4 railgun accelerators, 2 shields (probably not needed) and perhaps some point defense, skeleton crew and a lot of engines on reverse cost slightly over 500k and always took down the Marathon, it is so light, hence fast and easily steps out of range. That ship is engines strapped on railgun, not even one small laser. Excelsis under ai control demanded a lot heavier ship with a lot more shields, 700k, jump in, take one shot, receive one shot at 4+ density shields and flee, doable and that ship eats all AI ships I have found for breakfast, but beating Excelsis was not fun.