Engine is the 1st priority of course. I'd rather have a POS looking car that'll own on the track/ streets (or wtever u prefer) over a good looking sexy @$$ car that can't run.
O, and if engine isn't the 1st priority, i'd guess i'd have to go with SAFETY items.

 

Keep your car a sleeper bro. When you pop the hood make it look all OEM stock header and stock air induction. Poor a little motor oil on the side of the head to "make it look" like it is leaking really bad. Even let your racing competitors laugh at you and "act" like you fear them. When you go out street racing place some bets. If people think you have nothing they are more likely to bet they will win.
In other words do machined internal work to the engine so the true performance can't be seen. Such as cams and cam gears, machined counterweighed crank, titanium retainers and valve springs, aluminum fly wheel, P&P head work and honed intake, wet 50 shot of nos underneath the intake induction and right before the TB "Hide the tank in the trunk and hide the NOS injection button with a drill and tap through the shift ****",Electric radiator fan, aps underdrive pulley, 300zx/z32fuel pump and filter, z32 MAF hotwire, MSD ignition system, cheap 13dollar intake filter, EGR block off, coolan't bypass, advanced timing, cheap 21/4" exhaust, ACT stage 2 clutch, and keep your APEXI SAFC under the seat until it is time to race.
Don't forget to get good tires and a suspension to grab the road right before you say ready......set......holyshiaaat!!!!!!!!!!!!!!!!

 

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nice.. bump to BigVinnie