In 2003, the 6 came in 2 different body styles; the 4 door sedan, and the "sport wagon".
In 2004, they added the 5 door sedan to the lineup.
All body styles had the I and S models. The I model had a 2.3L 160hp I4 engine, while the S model had the 3.0L 220hp V6 engine.
As far as options, I think Mazda had a bunch of things clustered into packages, and I have no idea what these packages included, or what they were called.
However, I know most of the INDIVIDUAL options;
-Power driver's seat. Passenger's seat doesn't have this option... Which sucks.
-"Ground effects" kit (tiny lip kit)
-Bose system (I've heard it sucks)
-Navigation (I think)
-Fog lights integrated INTO the headlights. (The 6 had a total of 5 bulbs in the headlamp. Low beam, turn signal, fog light, high beam, parking light (above the highbeam)). If the car doesn't have the integrated fogs, then Mazda would just put a plastic cap where the bulb should go.
-Lower bumper fogs. -- I don't think you can get both the integrated AND lower fogs, it's either/or. For looks, I'd go with the lower bumper fogs.
These are all the options I can remember at the moment. I'm sure other people will join in with anything else.
However, I don't know how big your budget is at the moment, but, I've heard that the 06's are going CHEAP. Like, $17,000 to get an I sedan w/ rims and such.
If anything, you can use this as leverage when you're negotiating a price for a used 6.
Hope I helped.
PS- I think you could add options to the car, but I'd suggest you don't. We have a bunch of sponsors on these forums that'll get you OEM parts for CHEAP as hell. My fogs cost me $143 @ Showcase Mazda. My dealer quoted a helluva lot more.