[gothroach]

The low end Rigols make good entry-level scopes, and have a surprising amount of capability for the price.

As for learning resources, I came across a decent article on the subject when I was starting out (1), and most of the oscilloscope manufacturers have whitepapers on SMPS diagnostics, the Tektronics one I read a while back (2) gave a good overview. A lot of the whitepapers have a manufacturer-specific focus, but they still have good information that can be applied to almost any oscilloscope.

If you want to get really into the power supply and do high-side measurements you'll need an isolated differential probe, which can cost as much as an inexpensive oscilloscope, but for DC output measurement you shouldn't need anything special. Current probes are a lot more affordable if you're interested in looking at loads or current fluctuations/harmonics, but that's more useful after you've figured out a bit more what specific properties you're trying to measure.

1: https://www.testandmeasurementtips.com/test-switching-power-...

2: https://download.tek.com/document/3GW_23612_7.pdf

Edit: I forgot to mention that the EEVblog forums are a good resource also, but they sometimes aren't as friendly as they could be towards people just starting out.