Yes, you need a FPR to raise the fuel pressure to 4bar. However you dont need to raise the fuel pressure to 4bar, and TBH it creates more problems than it solves them.
Remember your fuel pump has to deliver fuel to FPR setting + boost, and the more pressure, the less flow... most pumps wont flow a lot past 4.5bar. Injectors are also made for 3bar base pressure, so they wont atomize correctly and will heat more with 4bar fuel pressure. that and gaskets will suffer ... Not worth the trouble imho, just get the correct sized injectors.
I dont know the KA24, they did not sell it in France. I am using the CA18DET. They come with 370cc injectors as standard and run around 170HP. They are maxed with 1bar fuel pressure on regular pump gas, which translates into approx 220hp.
If you get a nistune, adjusting will be easy and fast to do. There can be a 1-2 AFR variation between 2 e85 tank, so it just is a matter of seconds. The fuel map itself does not need any correction, you just need to change the K value a little bit ( although it is a bit more complicated as you have to change the TP scale and crank value too. But nistune can do it for you, you just need to use the "resize injector button" , put the old K value in the "new injector size", the wanted K value in the "old injector size", and nistune will do all the complicated things for you. You just need an AFR gauge and sensor)
As for injector size, it all depends on what you plan for your engine. Will you stay to 250hp ? Will you want more ? Just dont buy crappy redrilled ones
i think i'll go with 1000cc, so i get a nice margin for future upgrade.