for 300whp you can avoid a lot of this stuff!
Turbo (i haven't decided what brand or comp housing sizes yet, advices please? High flow the standard and you can achieve this. Just ensure you have an aftermarket dump pipe! will help alot
High mount manifold (i would need a new exhaust made up to suit new manifold yes?) not required
Fmic (was thinking plasmaman?) FMIC is always good and should be one of the first things to drop intake temps
Fuel pump and regulator (bosch 044 should suffice) Excellent idea and also walbro are also excellent value
Injectors (no clue how large i would need to go) plenty of bolt in options here depending on what you want to do
Forward facing plenum and fuel rail (probably go for a greddy) over kill IMO however does reduce the pipe work in the engine bay. Also plenty of options for this
Breather catch can (make the rocker covers look a little nicer...) this can be skipped as the factory breather option is pretty good as it is. Can add a tank however i doubt your car would require this at any point with the power you require unless you are going to track it
Cams (tomei poncams yeah..?) not really worth if for the small amount of increase in power to cost! spec ially if you keep the standard turbo and high flow it
Do i need a new throttle body? Are there good conversion kits avaliable? I've heard i need a metal head gasket at 300hp? But also heard if the head is torqued properly there is no need... If i do need the gasket, while the head is off do i get it ported and polished? And maybe shave 20thou off..? ( ) is there anything i am missing? Oil and water pumps should be fine ay? all this should be fine! no need to change any of that stuff as these are more than capable of holding the power you are asking for! no need to shave head or change head gasket
all in all 300whp is easily achiveable with next to no changes with what you got but as GTSTTRK stated there is plenty of info on here