[SpecB]

It's a difficult one - if you only go with the short engine (cheaper) and re-use your heads to avoid the AVCS 'issue' it will almost certainly end up more expensive as the block and your heads ideally need skimming/checking, add in new Head Gaskets (uprated would be the obvious choice) and the 'build' cost it will be more expensive than simply dropping an STI lump in complete.

I know you can run AVCS heads without the AVCS plumbed in but I'm not sure exactly how much you lose (I only had mine not active in the running in map) as the cam timing changes you will probably feel it most at the mid to top end as your cams will run in the low rev position constantly.

I'm sure you can drop an STI ECU into the WRX and repin the connectors using the tumbler wiring (good excuse to bin them while it's in bits) to gain the AVCS control. Shouldn't be that much more expensive.

The obvious thing would be to fit the STI lump with the tumblers deleted and run with out AVCS for a while then if you desire you can simply swap the ECU and re-pin without any disturbance of the engine etc. and get the car running to it's full potential.

Other thing to suggest - you don't need the STI inlet you can just reuse yours.

Could you find a breaker that will sell you the 'long' engine with inlet, injectors loom and turbo - may get a better deal that way, and even get the matching ecu off them (I think you need the key as well)