1) #1 for a reason! Find the noise - and fix if/as necessary before it becomes a real problem!
2) Most people consider a stock block to be fairly reliable at under 350AWHP.
3) If you are concerned about reliability then for far less than the cost of a new engine you can have a spare car. You don't have to swap engines to use it when needed. I have a 2002 Civic we bought for under $2K. I was glad to have it when I shattered my ankle and could not drive stick. I do realize that this may not be practical for city dwellers, but most city dwellers aren't talking about building their own engine - cause they'd be doing it on their kitchen table. OK, I built a VW on my kitchen table around 82
still . . .
4) Unless you really really want to build your own block, or you are already an Subie expert, you'll be better off buying a built short block if you ever want or need need one. Start looking here:
IAG Performance Subaru Short Block Built Engines for WRX, STI, Legacy GT, Spec B and Forester XT.