It's a difficult one Fluke.
Are you bored of the look of the car then?
Is it just because you feel you've modded it as much as you want?
Are you after more performance?
Or do you just want to experience something different (turbo, 4WD etc)?
Even though I have spent loads on my Zed I often think about changing it. It's not that it doesn't look great or go great now that's it's supercharged it's just that I'd like to experience different cars for different reasons.
Also without having my engine rebuilt or adding NOS to it I've kind of reached the best performance level I can get from it without throwing an even more ridiculous sum of money at it. I could get a few more Bhp here & there from certain things but it won't be the performance jump I experienced with going from NA to FI.
Now some of the cars I lust over are out of my current price range, like the R35 GTR for example. Others though I could definitely afford though like the Audi RS4 and Evo 8.
When I used to change cars it usually happened because I was chasing a better performance vehicle that I could afford and afford to insure. Nowadays the insurance isn't a problem so it's just how far does my budget stretch.
My advice would be to keep the Zed for the time being Fluke but stop spending anymore money on it bar the service items.
Start saving up and start selling some of the parts you don't need for the Zed & keep looking through Autotrader, Pistonheads etc to see if anything just jumps out at you that you'd really like.