Heavily considering jumping into the 818 project

I've owned nothing but Jeeps since I've been driving. I love them and but it's time I get something sporty and fast. I was originally going to get a 09-12 Mustang and upgrade it over time. A buddy who is also considering an 818 turned me onto this. I've worked on cars and such, but my garage just has your basic tools and such and I've built my Jeeps so I feel like after watching the construction video that I'd be able to do it.

I'm mainly wondering if I need a donor vehicle. I live in CO and 02-07 WRXs are expensive as hell. I didn't search much but I'd honestly be getting the engine/tranny rebuilt anyway along with cleaning up the other parts. Would it be an overall better option to just buy new? I know it'll be more expensive, but everything will be good to go and reliable. I'd eventually upgrade the suspension, brakes, turbo, and exhaust anyway from the donor.. However I'm not opposed to buying a roller and upgrading the parts later on.

I'm also iffy on putting a 2.5 in since I hear a lot of Subies from those years have head gasket issues and such. Will it not be as big of a deal with a new motor?