don't know much about turbo sizing,but you did say you wanted a new turbo.t03's & t04's
aren't new.very old designs.the gt25 mentioned would probably do the trick.something i was told by a friend years ago was that it didn't matter how big a engine a turbo was on,it'd still make the same horsepower,because it would still get the same amount of air into an engine.what was different was where it made it's horsepower,because the bigger the turbo,the more exhaust gases were needed to spin the turbine.so an a15 wouldn't start spinning a t03 until very high revs.this is alright in a drag car,but not in a street car.as for a t04,i don't think an a15 would have enough gas flow to get it anywhere near its range.you could probably get a good turbo off an sr20det,when the owner upgrades to something bigger.i don't know if there's a demand for these,as two times i've casually mentioned about getting an old sr20 turbo for something & both times the person i was talking to had one sitting around.