I would suggest sampling along a line at least the same as
the number of lines. ( a bit more if you can )
That should give a reasonable resolution.
PAL (Phase Alternate Line) doesn't necessarily have to be
used in conjunction with interlacing, but it usually is.
You dont have to worry about the PAL set-up for black&white.
Where interlacing is used,
the digital readout would have to be modified accordingly to
be shown on a PC.
But it should go straight to a television just as it came in.
(ps. sampling horizontally at a lower rate than vertically,
vertically being about 620 and fixed, will make the picture
look 'smeared' or 'smudged' horizontally.)
Best of luck with it, John