What type of mortar should be used for small mortar repair on a 100 year old house?
With a facing I would clean all the loose out and mix some but I have the lime and bags of clean sand and pick up some fresh cement. Since you are new to this I would suggest a pre mixed repointing mix. It is a little more expensive than mixing it up your self. Pre mixed is basically Portland cement, clean fine sand, hydrated lime, and water, I will use an add mix it helps to seal but I don’t know if it is necessary, I started using addmix after learning how to tile. 
The reason I asked about foundations is then a stronger type (n or s ) would be needed. 
For exterior above grade any grade n, s or m will be fine. But purchasing a premix just go to a big box store and find the pre- mixed repointing bag. Add water according to the instructions better to start with not enough water and work up. There are acrylic? Premixed that you put on and let dry but I have not used these some one else may have advice on them.