The v6 is gona be pretty pokie as it is but a decent induction kit and free flowing exhaust system from either the cat back or have it decat'd to really open her up (dont forget your need the cat back in for MoT time) after that I'd leave the engine alone. Simple breathing mods should wake the performance up a tad and also get a great soundtrack to listen to while driving.
As for body mods, there are a few kits out there and other body parts like front and rear spoilers but I wouldn't go to mad as the 156 has great lines you just want to compliment them lines not detract from them.
You could look at the high rise evo style spoiler which looks nice with the sports skirts that higher spec'd cars come with.
Either a nice twin pipe like the GTA has or a large oval tailpipe should fill the bumper cut out nicely, I'd avoid Jap style/custom/universal dustbins as again I'm not sure they would suit the clean lines of the 156
When it comes to styling its all personal taste and its your car and money so buy what you like
Get some pic's up in the pic section