EVERYTHING IS POSSIBLE IN SINS! thats why the game is sooooooo AWESOME!
So to answer you're question..the best way to give it the "unique" feel is just to create abilities and all unique to the race in which it will be added.
So for example: An uber TEC ship would have a great AoE regenerating ability, a nice little variant of missile barrage, how about it having a mobile planet destructor (IE. giving it a nerfed Novalith ability).
Same goes for stats...
As for otherways of making it unique:
-Give it a nice name and a meaningful and inspiring definition (IDS NAME and IDS Description found in entity files and that you change in STRING file).
-Create custom sounds and add them to the ship (fairly simple, just make sure you have the right file format)
-Finally....MAKE A CUSTOM MODEL! For that download XSI Modtool (google it..its FREE). Thats the tool the developers used for the game and its extremely user friendly (IMO)!
Any other questions..feel free to ask!
-ShadowMastif2468-