Hey there! I would strongly suggest a tdo 5 turbo, evo 8/9 stock exhaust manifold, evo 9 front mount intercooler, and sti pink 565cc injectors. With ecu flash tuning, this should get you a decent 1.4-1.5 bar boost and 300+whp. With additional high cams such as gsc stage 1 or hks 272 and adjustable cam pulleys, you can push for 330 - 350 whp with this setup, boosting approximately 1.5-1.6+ bar.
If you want to stick to 1 bar of boost, the stock tdo 4 turbo with stock top mount intercooler can push about 260-270whp max with about 1.2-1.3bar boost. this would still need a larger fuel pump, fuel injectors (sti pink are great) and fuel regulator as well. the tdo4 is maxed out at such a boost and might not last very long though. The exhaust manifold of the tdo4 turbo is also much less resistant to heat and corrosion as compared to the stock evo 8/9 exhaust manifolds (for tdo 5 turbos).
Regarding the best bang for buck, I guess it depends on how much whp you are looking at. The evo 9/8 tdo 5, stock evo exhaust manifold, front mount intercooler, injectors and hks 272 cams with adjustable cam pulleys are the best bet in my opinion. Built to last, better fuel economy and a huge difference from stock tdo 4. Also in both setups, you'll need a boost controller (either manual or electronic are both fine), larger fuel pump (I use AEM but I heard DW is great as well), larger fuel injectors (sti pink are great for direct plug and play - note that these injectors need to be high impedance types or you will need to add a resistor pack to prevent frying your ecu), and fuel regulator (such as sard, tomioka). Tuning can be done directly on the stock ecu through reflash with a suitable software. Do also be mindful that the tdo 5 setup might require a slightly larger downpipe and maybe decat to avoid choking. The stock muffler is sufficient for airflow if you can operate it to make the flow smoother. Don't have to necessarily use Apexi N1 which is rather noisy in my opinion.
Hope this info has been useful!