if you are going to mod then i say do it. seems much more cost efficient to me to buy a less exspensive Z and change what you are going to change rather than buy a more exspensive Z and then still have to change whatever parts out that you want. it's hard to wrench on something that isnt broken. My current Z that i'm building i got lucky with, as the upper idler pulley fell off on the previous owner while he was driving down the road. the Z's body is straight lowered already has a few upgrades on it. I bought the car for 1600 bucks which in my opinion is a steal seeing that the body only has 90k. I had already planned on doing a major overhaul to my previous Z but never got around to it before I totaled it. But now because this one isn't running it's a LOT easier to be patient and upgrade everything as I want. In the end it's up to you and what you want to do with your money.
