Combining a VF22 & front mount is probably going to leave a laggy car. Would definitely be worth driving on an extended test drive to see would it suit you.
New pistons would suggest an engine rebuild but no mention of this or who might have done the work. No mention of who did the mapping either. Mapping with those mods would be absolutely essential.
Gearboxes in uk spec cars are of the "softer" variety. It would struggle to deal with 300bhp on a daily basis in my opinion.
Paying big money for a wrx, you're well into STi territory there at the price listed.