Solid advice above. On a crop DSLR a 50/1.8 can feel tight indoors, so a 35/1.8 is usually the friendliest pick, and used or refurbished copies from your brand or Sigma/Tamron often land well under your budget. In the meantime, zoom your kit to the long end, keep your subject far from the background, use single-point AF on the near eye, keep shutter around 1/160–1/250 to avoid motion blur, and when you get the fast prime try f2–2.2 for sharper results with plenty of blur.
