How much power are you looking for ultimately :?:
For 400bhp ish I'd say go with the turbo Ashills has (jgybb37) its good for 400bhp and is pretty much a direct fit on the standard manifold and dump pipe etc though you will need to slightly modify your old turbo oil drain flange to fit and you'll also need to make up new turbo intake piping and outlet piping
but wait alittle as jgy are redesigning it for better spool up 8)
Or of course theres the HKS GTRS which needs even less mods to get it to fit
For 400bhp+ theres a endless choice really but you'll really need to change the standard manifold as it becomes restrictive above 400bhp which adds to the exspense
I think the GT25R has a 4" inlet iirc :?: which will not fit on the standard manifold without either welding the flange on the manifold at a different angle or spacing the manifold away for the head and fitting longer studs etc
Then theres whether you want to use a internally gated turbo like the jgybb37 for easier fitting or a external gate :?: which would be more costly as you'd need another manifold or the standard one modified for external use aswell as custom downpipe/screamer pipe etc