Re-Pointing Fieldstone Foundation
I have recently researched this for my granite foundation. There's quite a lot of info on the internet on this topic, but here's what I can answer based on my experience:

You will only need the most basic tools: a throwel, joint filler and somewhere to mix the mortar. I also recommend getting masonry hawk to hold the mortar while you are repointing

This can easily be a DIY project as there's nothing complicated or requiring expensive tools. It will definitely take a long time. I usually work on my foundation just a few hours a week and there's still quite a lot left. The process is pretty tedious but I find it relaxing. It is probably best to work in small sections so you do not destabilize the whole foundation. I typically remove crumbling old mortar with the joint filler, vacuum the cavity, spray with water and fill it with the new mortar.

I think fieldstone is pretty soft, so even type N mortar might be too hard.  You might want to look at type o or type k

If you decide to hire someone, I would make sure that they know and understand old masonry and techniques.