Which ECU?
 

I have a standard my98 Terzo, 70k on the clock

I'm just starting to buy bits for modification,

FMIC
VF34
550 injectors (finding hard to get)
Walbro fuel pump
FPR (not sure which yet)
decat exhaust turbo back

I will be putting the car together in one go when i have everything then remapping to suit.

I was looking at the Apexi fc which sounds great but there are so many ECU's to choose from I don't want to have to change it later on?

I'm looking for around 350 bhp

is the apexi a good idea or should i go for something else?

(the gearbox will be getting changed soon too, 6 speed)

Simtek seems to be the right way these days.

However, of course, apexi power fc will also do the job if you can get it cheap enough - but Simtek seems to be a bit more modern, full of features to control this and that :)
Not a problem with your model - but very good thing about Simtek is that it's MAF-less and allows my99/00 users to get rid of fragile sensor, failing pretty often.