Hmm, in actuality, it looks like you have one intake (case fan up front) and one exhaust + PSU + barely nothing, so your system really isn't so imbalanced for airflow as we may initially suspect. (Please correct me if I'm wrong in understanding the top exhaust to be via the PSU.)
As for good articles, an excellent place to begin is in the cooling stickies section.
As for a side intake, yes, you'll need to go through metal. It's not
quite so bad as you might expect, but it does require time and patience, especially if you want a
round hole without a lot of scuffs.
It can be done very patiently with a dremmel. More ideal would be an electric hole saw. I don't have one myself, so I can't comment on specifics there.
My personal recommendation would be to put a fan in your side panel roughly directly above the HSF, and I would go 80mm or larger. It is also a good idea to cut a piece of cardboard that fits your side opening (same size as your panel) and practice the fan placement on that first. That way, you can prototype a bit and find the best location for that fan.
Best of luck!! -- Paul