I wonder why you even ask for suggestions when you shoot everything down.

18+ look better with a drop, fills the wheel well, but no you want to keep your 17's.
Don't go to Altezzas, IMO they look like hell and ruin a car's rear. I don't know about those covers you have, maybe the Euro ones with the amber corners.
You want to keep the flames, fine, I don't like flames on slow Jap cars.
A tach would be nice just so you have one, you don't need a monster 5" tach like some guys do, just a nice 2.5-3" setup on a pillar gauge pod and you can throw in your turbo gauges if you ever decide to make your car halfway fast.
Z3 fenders are played out way too much and they are gay anyway. Some guy has some with the vents like the Dodge Viper and someplace (Stillen?) sells fenders that are more like the M3 style. I like those.
Paint all your hood or don't paint it at all. 2-3 colors on the hood is going to look retarded.

Luke

1998 Mustang GT 5-speed
2002 Lone Star Shelby Cobra Replica