It's a relatively easy mod and definitely one a beginner could do. The problem is it's pretty easy to lift pads due to the age of the system. I've done a couple and the boards of the systems I've worked on were far from pristine shape. I'd recommend finding a practice board to work on to practice your soldering on because excessive heat will increase the likelyhood of you messing up the board. Find something cheap at a thrift store and practice running fly wires. The mod is all through hole which makes it pretty beginner level in terms of soldering skill and all components coming off are scrapped so you don't have to worry about damaging them. If you lift a pad the repair is easy as all traces are surface traces. Cut away the mask to expose the base metal of the trace and bridge the solder between the wire and the trace.
As for messing up the cosmetics of your box, you do have to cut it. It's pretty easy, just some holes to mount the composite jacks, so if you have the right tools it shouldn't be an issue at all.
Edit: I would probably recommend getting a kit rather than trying to build the circuit from scratch for your first mod. With a kit and a decent guide you should definitely be able to do it.