Last I looked, the cheapest Mk1 Nismo on Autotrader was 28k, 63 plate, so even with a sizeable negotiated discount, it'll probs be @ £26.5-£26k. That's about as much as you would pay for a pre registered/1 year old standard.
Anyway, HFCs, I had slight probs with my last 70, think sipar had too, of a CEL, after several resets, I took the HFCs off. They're not needed for a louder noise on a lot of systems for the 70. Mine being an example (Ark GRiP) as for adding a few ponies, hmm, if they did, I never noticed, my jury is out on that one.
£1500-2000 for an exhaust is way too much to spend, even my Akro wasn't near as expensive as that, include the HFCs and yeah maybe nudge toward £1500, but for a meaty, beaty, big and bouncy exhaust, budget @£1k-1100, much less with a custom . There might be second hand ones out there as well, taken off when owners move on.