There's no such thing as a 300E V6. It's a inline 6.
Besides that, you will want to check the tranny for flaring, 'clunking' when shifting between R & D and propably other things indicating a bad tranny.
You want to check that the engine is smooth idling and accellerating both cold and warm.
When you test drive it, try putting the shifter in 2 or 3 and accellerate to near redline, let off the gas for a few seconds and then gun it again while you look for smoke in the rear view mirror. (worn valve seals/guides)
You should check all of the bushings and control arms in the suspension. If they haven't been replaced they properbly will need to soon. -Also indicated by the car tramlining bad on the road.
Also check the engine mounts by shifting between D and R while looking at the engine (hood up). Does the engine tilt when shifting? = New mounts.
Make sure the sunroof is working properly. It's insanely expensive to fix if broken.
If there's any rust in the car, you will find it at the buttom of the doors, under those chrome fender flares, or in the trunk right below the tail lights.
Good luck
Freestyler