So you have an engine. Now you have no means of mounting it, so you need custom mounts. Now you have no means of mounting the transmission, so that's custom. Now you need a new custom driveshaft to put that power to the rear wheels. Now you need to upgrade the diff because the R160 diff it comes with is tiny and is not meant for that kinda power ALL to the rear wheels. Now you need new axles.
So you've got the engine physically mounted now, great. Now you gota turn it on. Now you need a new wiring harness that'll adapt to the chassis components and the engines ECU which hopefully comes with the engine. Oh it doesn't have the ECU? Now you gota learn megasquirt and deal with all that shit because an actual standalone ECU that'll do what you need it to do is well over 5 grand.
Now you need new gauges because the ECU you have doesn't want to talk to the ones in the Subaru unless you can pull off some slick electric engineering. Since you just dropped THOUSANDS on this damn car, you're gonna want some suspension components to go along with this because you'll be damned if this heavy beast can't really corner for shit.
Now it's 10 years later and you're staring at this damn thing which doesn't run either at all, or poorly, and you're wondering, "Why the hell didn't I just change the head gaskets and buy a bigger turbo for the stock engine? I'd be done years ago!!!"
More time goes by, you finally kick your alcoholism that you needed to cope with the neverending torment of this unfinished project and you get serious and put in some serious money to just get the damn thing working. It finally turns on, everything is working just fine. You go down your street...and you realize it's not any faster at all then a freggin stock Forester turbo. Darkness sets in.