I assume the EJ257 block will be mated to your EJ20 heads??? As long as you are working with a pulled block, then it's an easy matter to replace (with OEM) oil and water pumps, thermostat, timing belt AND tensioner. It's the tensioner that goes bad, and it's not cheap. OEM gaskets and seals are more than adequate. For the price (about $170) ARP headstuds are a good idea, though not necessary if you don't plan on pulling big hp and trq.
Here's the thing, though. It's the nature of the beast (all of us WRX and STi owners) to want big power out of our engines. Sooner or later, and probably sooner, you are going to want your engine to be all that it can reasonably be. But, okay, for the sake of argument, let's say that you are that one WRX owner in a million who does not want big power out of his engine.
In that case, why not stay with the 2.0 block? It revs faster, and you won't have to worry about getting the 2.5/2.0 engine to breathe properly. And if you keep it under 300whp/270wtrq, you just might be able to keep the stock trans alive. Believe me, as a daily driver, these numbers will definitely curtail the desire for coffee on your way to work in the morning. AudioSlave at unnatural decibels and a 300whp WRX will clear the 'ol cobwebs out, that's for sure. Of course, if you are a Jack Johnson kind of guy . . .
But back to the hybrid:
The easiest and most cost effective way to get the hybrid to breathe properly is to add cams--which in turn means stronger springs and retainers and, EQUALLY important, a bigger, meaner turbo. It also means a better fuel rail, bigger fuel pump, aftermarket boost solenoid, bigger intercooler, uppipe, downpipe, exhaust, and, of course, injectors. The stock 2.0 injectors are fine. Witch Hunter will modify and flow test them to 815cc. Also, as long as you stay under about 500whp, a reflash is all you need. The OEM ECU is pretty amazing.
A lot of people are P&P-ing the heads. It's not necessary unless you are looking for obscene power. Take out the tumblers (the manifold is off. It's easy to do, and will help air flow). Have your heads cleaned and make sure the valves seal properly. Have a professional with a glass-bead cleaner do this, and have him put on the springs and retainers (about $250 to have a professional do all this). Cams are easy to install (you've got the engine on a stand), but you will probably need to replace all or most of the shims. You might want to have a professional do this (add about another $100-150 for the professional). Triple check your measurements if you do it yourself. No. Quadruple check your measurements. Your local dealership will have a list of shims, and the Portland warehouse will have whichever you need.
By now, if you've done all this, you are asking yourself why you didn't crack the block and put in aftermarket pistons and rods (the engine is still on a stand). And you are starting to ask yourself just exactly how much power this thing is going to produce, anyway. For sure, it's going to be A LOT.
Which brings us to, you guessed it, the transmission and the clutch.
As I said, why not stick with the 2.0 block?