Paint or stain 150 year old door?
Like @Fizz says- the answer depends on getting all the paint out.  Stain will will make the paint spots stick out like several sore thumbs.  

A chemical stripper will help, but it is still going to be a lot of painstaking work.  I would leave shallow scratches or pin holes that are paint free as is.  Deep scratches or holes that have some paint stuck in them would get the putty treatment from me.

My gut tells me that the crack in the middle of lower panel is going to be visible no matter what you attempt.  To a lesser degree and depending on how dark the stain will be, the same goes for the gouges around those screw or bolt holes.  Even with a good filler, the ghost outlines will still be there by virtue of the grain being interrupted.

The ghost outlines might be one of those things where you will always notice it every time you walk by, but the average non-obsessed person won't even see it unless you point it out.  If you can live with that, go for a stain.

If you go the stain and filler route, do the research and get something suitable for stain and exterior application.

Good luck- the door looks great.