you'll have a number of different choices on front control arm bushings (assuming that's what "solid front bushings" refers to). my mechanic (who also race preps and races 318s) suggested the e30 m3 CABs (still rubber, but harder and he said he's never seen them fail - i also have mine as a daily driver). there are urethane bushings, centered or offset (e30 ones are offset, believe someone else commented somewhere that it makes it a little more stable - hopefully someone can explain this more in simple terms and/or correct me if what i just said is wrong) ... i think delrin ones are available too.
the "butt strut" (not the official name, but its nickname) from strong strut (
www.strongstrut.com, look for Z3 rear strut bar) would be my recommendation for "rear undercarriage brace." not cheap, but there are some people who would consider going in on a group buy at some future point (maybe 6 months from now for me personally) ... i know L84thsky was interested.
if you choose lowering springs, i would also very much consider putting on a camber kit. i know bavauto.com and bmpdesign.com both offer them. bmpdesign for some reason doesn't list the front kit for our car even though it should be exactly the same as the other 6-cyl 3ers, go figure. front and back would be about $600. otherwise you'll have really nice negative camber like me
you might want to consider stiffer springs that offer little to no lowering (if you can find them) or a coilover kit that offers a highest point of adjustment in the perches that results in little to no drop. another thing to consider in lowering your car ... you very well might not be able to get under it any more! i used to be able to change my oil without raising the car at all (yes, on a flat garage floor) and now i have to get up on ramps. oh yeah, my regular car ramps wouldn't work any more either after i lowered it, even with the extension kit i got which makes the angle shallower, the front bumper cover would STILL hit. had to fashion mods out of 2 x 10 pressure treated wood to make it all the way up to the top without hitting. AND i have to be careful of many things i didn't before now because it sits lower (speed bumps, driveway entrances, ice/snow chunks during winter that are of any significant size, etc). personally after all this, i'm wondering if lowering is really worth it. just my humble opinion.