Turbos; T18 vs T19 and other components

[Juan62]

I am in the process of upgrading my engine from mostly stock to performance. I need advice on whether to use a T18 or T19. Also, what downpipe should be used; angle or straight flange? I also planned to upgrade my ECU and possible a water/meth setup. Do I need to change the fuel injectors too?
Current parts installed:
IPD HD TCV, IPD MSD Ignition coil, IPD Performance 8mm spark plug wires, IPD (RIP) Intercooler kit, IPD short-air ram kit, and Snabb fresh air pipe.

In advance, thank you for your input.
Juan