A Retribution as AdMech Battleship is indeed a better investment then the Ark itself.
At least in my opinion.

Lunars are a good choice. I also run a Gothic. Five lances is quite formidable as support.

And the Defiants are poop. I ran one. I lost. I changed it to an Endeavour I started winning with my AdMech. For ordnance support one Dictator will be sufficient. You won't win the ordnance war but with all the extra turrets have enough defence in that regard.