I'll start with the negative (Sorry). I don't understand why you didn't get a Spec-R? Unless you get some cracking prices on parts, it just seems that mid to long term you are going to come out worse. I'm talking financially not just about the badge.
I know what I'm getting into.
S15 SR20DET is more powerful than the S14's variant for a start (so S15 engine's have a premium). I assume you are going to need AT LEAST a GT28RS to get to 325bhp.
Motor is the same, extra power is just from more boost and better IC. I will have a nice turbo/FMIC/nistune anyway. Btw you don't need a disco for 325, the s15 t28r will do it. I have some plans for something more experimental though.
You'll need all the electronic gubbins as well, you might need a custom exhaust.
Zenki S14 ECU with nistune board, easily done. Exhaust I already have, but only the rear section is s15 specific.
Different diff.
Helical not really for me, it will have a clutch type diff no doubt about it.
More bracing (Drifters comment that the S15 Spec R feels like an S14 with a cage).
They are slightly delusional IMO. As I said though it's all bolt on, if I can pick up the bits I will fit them.
Better brakes.
Sumitomo 4 pots are 10 a penny, not a big deal at all.
Better gearbox (Some think the 6 speed is weaker though).
It is weaker, no doubt about it. I don't care about 6 speed nor do you need it with a torquey engine.
Has a body kit (Which also means you'd probably need a paint job)
Actually Spec-R does not come with any aero as standard, the options are exactly the same as Spec-S
Let me say here.. I LOVE S15's. I've had S13/S14a myself, a long time member of SXOC, a (probably 'ex' now lol) committee member is sitting about 15 metres away from me at the moment. Are you sure you worked out the price difference correctly?
Quite sure. Oh and I've been on SXOC, the place is a ghost town.
BTW: Look to sell the N/A engine to someone in the Primera Clubs.
Already have an interested buyer, also there are significant differences between the RWD and FWD SR's
All that aside:
I'd work on suspension first. Coils (or something milder like tein replacements), locking collars (MASSIVE difference) and possibly some ARB's.
Brakes next. Obviously you are in 300ZX circles, so that should be a nice cheap upgrade to 4 pots.
Engine last. You might actual find it's cheaper to get something like an LS1 V8 in there.
That is exactly the plan, Full adjustable arms is pretty much a requirement though, and a thirsty V8 is a no-go for what will be a daily driver. Plan/order is irrelevant though, it won't get put into use until it's fully up to spec.
Make a plan Clarkey... I've lost count of the number of years said committee member has his S15 sat in a garage undergoing work, dipping, resprays, more stripdowns, more work, quick release fronts, tubbing arches etc etc... and not to actually drive it.
Also.. pics.. I demand more sexy pics.
They will come when there is more progress.
p.s. Forgot to say, get that underside protected asap, or your investment will be looking pretty bad really quick. (I'm guessing you have done this before.. but just in case).
Waiting for my delivery from Bilt Hamber