Units of psd_welch are (amplitude ** 2) / Hz, where amplitude will be the SI unit for the channel type (V for EEG, T for magnetometers, etc).
For keeping track of channel names, one approach is to store raw.ch_names (or epochs.ch_names or evoked.ch_names) in a list, then use mne.pick_info to get your pick indices. You can then use the pick indices for both the PSD function and for indexing the channel name list.