if your looking for big power dont get the z, if your looking for traction dont get the z, if you dont like a rough ride dont get the z, really the only advantage the z has is rwd(its my favorite

) and lower center of gravity(but very minor) the wrx and sti's are amazing platforms(yes ive driven and worked on both like i said i like rwd only reason i went with z was because it was too good a deal to pass up)
the 03-04 is actually the lightest z's produced go look up the curb weights my 03 enthusist was 3100 lbs from the factory(or mayby 3150? idk its several hundred pounds lighter now and easy to get 100-200 lbs off with standard upgrades.) transmissions are literally the most powerful manual transmission in the import world. stronger then both the getrag supra trannys and the 300zx trans, its only down side is the older ones had syncro issues just get a cd009 revision trans and your fine.
the biggest weak link is the rods in the vq35, dont expect more then 400 hp reliably out of the de and 500 on the hr motors and your going to be looking at big bucks to upgrade.(i spent ~5k ish on used pauter rods, wossner pistons, fluidamper, arp stud kits, machining, bearings, head gaskets, and cosworth oil control baffle and i got very good deals. unless you have the hookup expect to pay around 6k for the same, mayby a little less machining is rediculous here. depending on power goals eagle rods might work i wouldnt go above 700 with em though.)
turbo kit wise if you go that route expect to pay 4600+ for a new one. i wouldnt do any of the top mount kits personally i would go mid mount or twins, stay away from turbonetics and sts for sure both have huge problems on this platform. clutch you can go southbend for a cheap option, a 100% absolute must though is the rjm clutch pedal bracket its one of the best mods for the platform. engine management i would say get a old haltech for the cost of uprev+boost controller you can get the harness version haltechs.
it is not a cheap venture though expect 10k for a mild power build. for 800 to the wheels(cant wait for new clutch to get broken in lol) ive got about 28k invested now plus the cost of the car and still no brakes or suspension although i could have shaved that a little but i like to tinker. ALOT more if your paying someone else, do the same as i have done to my car minus the tinkering i would charge probably about 40k or so although there is alot of custom work done.