Regarding restoring the original firmware, check
this out. It describes the procedure for PRS 505, but for 300 it's very similiar.
Fsk & Root partitions from 1.0.00.24180 are
here and
here
PRS+ 300 installation which you could use as a base is is
here.
Note, however, that these are dumps from PRS 300 bought in Germany. (it supports 4 languages English, German, French, Dutch)