Personally I think "overkill" is moderately appropriate when choosing a PSU for the long term. My understanding is that most current high quality supplies have a fairly wide efficiency band. There is absolutely no reason to load a PSU to 90% in order to make it efficient rather than say 60%. It will only draw what's required. Most high quality ATX supplies will run quieter when not fully loaded. There are tangible benefits:

- A higher quality supply will usually have better, possibly 16AWG cables to connect to the miner. Lower cost PSU may have lower quality (i.e. 18-20 AWG wire) cables.

- Generally they are more durable, and withstand unexpected demands better (temperature, etc).

- Should have better resale value later.

I expect that used Server supplies are very available, and can work well, though they tend to be noisy.