Stainless is nice but not needed. All decent mild steel exhausts are "aluminised" and dont usually rust on a diesel. What i noticed with the DEA exhaust (when i was cutting and welding for custom turbo) is the bends shrink down to almost 2.75". Not really a worry, but worth noting. The tube is aluminised and the muffler is full stainless - quite good.
My 2.5" redback zorst (still for sale, make me an offer - a six pack and pick up will work) The bends all stay true 2.5". The muffler isnt stainless though. No rust after 10 years daily driving. bit of surface rust on the flanges and welds where there's no aluminium coating, and thats about it.