What kind of mortar should I use to point a fieldstone foundation?
I have been working on my field stone basement as well (170 years old), and here is what I have learned:

Having hired an actual stone mason (that's right; they still exist) to repair an area of wall that was very decrepit I learned some things that are helping me now, and I have also learned that some of what he was doing was not necessary.

I have begun my tuck pointing by chiseling away the loose mortar; sometimes there is a lot, and sometimes just a little (sometimes I can clear a cavity inside the wall bigger than a softball, and sometimes I have to chisel away sound mortar to have enough of a keyway to hold the new). This process starts by actual, but gentle, chiseling, and ends by scraping with the chisel until I am satisfied that any mortar left is sound. (Old lime mortar is more like scraping very hard playdough that the kids have left out accidentally). I chisel this away strategically, never completely freeing a stone from the wall, if I can avoid it, and never removing to large an area at a time; as I progress, I work from and join into the areas I have recently completed, so that the wall is well supported. Then I vacuum out the cavities completely before I begin mortaring.

I am using a type N mortar mix (already to go; just add water). Since this is about 25 percent cementitious material (lime and portland cement), I am adding sand to bring the percentage down to about twenty percent, to more closely match the hardness of the old lime mortar, although with all the conflicting advice regarding this, I doubt this matters as much as some say. The sand, however, makes the mortar a bit more workable, and enables me to pack it into the crevices a bit better.

I am using a trowel and tuck pointing tool to pack the mortar thoroughly between the stones, and completely fill the cavities (I mean COMPLETELY). Then, after curing for a little while (till the moisture shine is off of the mortar), I use a damp paint brush (really cheap one for this) to brush the mortar joints smooth, and to bond all the edges thoroughly to the stone. The result is pleasing to the eye, and completely sealed. After this, the stone mason advised me that a parge coat (completely coating the wall) is unnecessary; in fact, it seems to me that if repairs are needed in the future, that coating will make them harder to see early on, and complicate the repair.

I hope this helps. My repairs are progressing nicely, and look just like what the stone mason was doing.