Basically this is like saying "I want to build a mean GTR" and starting with a gte shell
-if- you had a written off STi you were swapping the running gear from, then sure it's a great plan and makes sense
or -if- you were starting with a rare or odd shell, say a 2dr GC you got for $500 or you found a repairable late model shell or something, then it's worth it cause you end up with something worth more than the sum of it's parts... Ditto for if you're putting an unusual motor in, say ez30 which is a (near) bolt in proposition
But realistically, I imagine you're going to pour 10k into what will ultimately end up being a grunty V7 shape not-wrx?
You could flick yours for say 5k, buy an sti for 8-10, then these days another few grand could net you a 300kw car (spent wisely)
And it'd have brembos, all the cool bits, AND resale value
Not trying to be deliberately cynical but I hate seeing money get thrown on reinventing the wheel