Thinking about building another car and want to know which Fireblade engine is best and likely costs?

It depends how bullet proof you want it to be Nick.
If you go for the 08 onwards with an ab billet sump,ab mods,ab headers,power commander and digital dash you'll be edging towards £4.5 -£5k I'd imagine possibly more. But a lot of the Westfield sprinters are running them with great success as are several of the rgb racers who are all very quick as well.
Mark ansons selling a beauty at the moment but if your aim is to build then it's probably not for you. But you could always take it apart and rebuild it for less than building

I've got a good 2004 GSXR 1000 K4 motor for sale if interested? Just broke the bike so have full running gear available too if you wanted.

you really want a very late 08 or newer CBR1000RR motor,
The early 2008s had issues with the cranks,

I picked up a nearly new 2012 motor with everything needed to make it run plus an AB sump, AB water pump delete kit, AB airbox mod, HRC clutch and all the resistors and voltage droppers needed for an absolute steal from the forum but AB do the complete pack for 3500 which maybe plus vat.

On top of that you'll need a power commander,electric water pump, and oil cooler kit, It's not a cheap install but it's proven to work and be reliable,

I bought a chassis kit with mounts for an 08 on blade motor and had the exhaust ect so it made sense to go CBR1000RR.
If starting from scratch I'd be looking at the R1 5VY engine, You wont go far wrong with 1 of those.

And an ab exhaust to help it breathe!

This does go well!
https://racecarsdirect.com/Advert/Details/68711/westfield-megablade