Your best to go on
www.sxoc.co.uk/vbb for the best answers to your questions. Its been asked many times on there
If you want 300bhp this can be achieved on a completely stock engine with a decent T28, exhaust, air filter, 440cc injectors and a Z32 air flow meter.
The Z32 afm isn't a restriction and has seen up to 500bhp on some DET's.
I wouldn't consider steel rods unless you are certain you want to break 400bhp. To break 400bhp you will be looking at alot more money for all the additional parts. It will start becoming a different game.