ive done a couple wrxs in my time, easy as once you know the process.you say its single but just in case heres simple way.FIRSTLY you need a small socket set with extensions.remove air box an half of piping(you can do this without removing the box but for 5bolts its easier just to remove it), remove water spray bottle, coil bolts only need to be unthreaded by about 10-15mm an then you can pull off.undo sparks.put in new sparks an tighten to required torque.reverse everything else i said. You shouldn't ever need to take battery out(i never did/have) it would how ever make things easier for you. the biggest problem I had was that the extension I had for socket was the perfect size to get jammed up on the chassis rails apart form that its pretty straight forward remove everything in way, put back together.make sure theres no intake pipes leaking or not on correctly as it will cause problems with boost
also should state thats on the older styled 90degree bend turbo subarus. so wrxs upto v2 an the early legacys , might be slight changes on newer ones but its really not hard at all.
I honestly Don't know why everyone makes a big fuss over these engines being SOOOOO hard to work on, bunch of whiners IMO, I mean sure its not gona be a simple ford escort change but its really not hard an if it takes you over an hour you need to smarten up or its your first time haha
also yes to getting good plugs, its a job you don't wana be doing every 6months an suby wll chew threw cheap plugs fairly quick