The r154 is the manual supra trans it's a 5 speed. Found in some mk3 supra's and can handle everything you can throw at it. You heard right about the 2jz having a forged Crank. The motor you already have can handle a lot of abuse. Boosting one is easy and reliable on a good tune. If I had a Is300 I would boost the stock motor. The trans swap is straight forward. Peddle, master cylinder, clutch line, slave cylinder, trans and driveline. Get a different ecu and if possible just get a standalone.
For the turbo swap get the exhaust manifold, turbo, piping and intercooler, injectors and fuel pump and different ecu. Not too involved and its worth the investment. I have seen Na-t 2jz make upwards of 500hp with stock internals and no problems. Good motor to start with.
Do a manual swap if your going turbo. I don't know how reliable the auto trans is with double the hp. Although Twincharger has a auto for his 1jz but I don't know if you have the same trans. Manuals are just more fun anyway. Good luck with your project. There are plenty of us on here that can help.