As someone who would love to get into street photography, and has an old NIkdon D7100, what would you recommend is a good lense (not model, but focal length, zoom, etc) for street photography ?
Prime lenses will have larger apertures that can give you more creative options.
How close do you want to stand? Indoor/outdoors? What are you planning on taking pictures of? D7100 is APS-C, I find that 50mm (~75mm ff) on APS-C doesn't give you quite enough room indoors to take photos. So you might want a 35mm prime or a zoom that goes down there. If you're planning on taking portraits you don't want something too wide (~20mm and below can be good for real estate/architecture) because it makes people look weird.
Most everything else is dictated by how much you want to spend and how large/heavy you want your camera to be.
Personally I have a 35mm f1.8 on my camera and am happy with it, I use it for family outings, a lot of portrait-level shots and just general "hey we're at the museum" kind of photos.