Welcome to the forum tbg. Your experience is unfortunate but you have the right attitude. You are obviously going to learn a lot more than you might have bargained for. Sounds like you have already done some good research in looking for clues to what might have happened. Removing the left cylinder will be a good place to start but you can be pretty sure that based on your description of the oil filter that you won't find just
a blown headgasket. Once you dive in and show some pictures or even before that you are going to get a lot of knowledgeable advice here. I'm working on my own 82 RS basketcase so I'm looking forward to following your thread.