I agree, do the mods and such on the old one. That new one sounds like an incredible find if it only has 15k miles! Typically with mods you end up going into the motor and buffing it up anyways. No need to do new engine work on a new engine, go for the old and redo it!
Unless you are going for a showroom type look for the new one, then obviously drive the old and keep the new low miles and protect paint and such.
04 IS300 MSM LSD Manual
-XS Power Turbo kit -Emanage Ultimate Tune
-SSR Type C 18x9.5/10.5 +20
-TRD Coilovers -TRD Sway Bars -CDD Delete
-Plastidip black Handles, grill, and stripe -
-Black Housing headlights and tailights