I sympathise with what you mean around paranoia - try driving a Stag as your other car! Any single twitch, clunk, smell (of which they're are lots) and strange noise has me on edge. Google "Stag paranoia" and you'll see what I mean.
I suppose when it comes to the Z though I've been spoiled - mine was similar (just over £6k for a vgc JDM on 58k miles) but coming from a '01 Fiesta that I've had for 8 years and that has done over 215k miles (200k in my ownership) I have a fairly healthy respect for modern cars and their engines. I think for me bodywork is the main thing. If you're even vaguely handy with a spanner most bits can be replaced cheaply, as long as your own time is cheap and it's all right to be off the road. Engine transplants are my limit though :-) It's the bodywork that I think is most important as although it doesn't alter how the car runs if left untreated it very quickly escalates from below a hundred for a 10-pence patch to a fair few hundred for a much bigger patch and, unless you have a proper booth and skill, spraying to get a decent match is hard. Not to mention welding patch panels (at least there's no lead loading, mind).
My last thought is that actually 8 years isn't an old car at all - up until the scrappage scheme and the finance schemes kicked in this last year or so 8 years would have been a fairly young second hand car!