buy the newest technology
Almost all of my current gear was purchased new (except for 2 subs) but I have had good luck purchasing used gear over the years as I kept learning what gear sounded best to me.
Relative to digital - I agree with you because that segment of the industry is still moving fast although it has slowed to the point where there are some good deals on good-sounding first generation used stuff.
With regards to other gear, I would be ok buying used and do not perceive big differences in the past 10 years or so, except at upper price points. There are some improved parts and materials that have resulted in some manufacturers upgrading their lines, but someone can still assemble a good-sounding system with used gear and speakers.