Spkerer, for purely visual spectroscopy, a good strategy would be to look for a used Rainbow Optics spectroscope, which combines a 200 lines/mm transmission grating with a cylindrical lens to spread the spctrum's width for easier observing. The Rainbow Optics unit (designed by Jim Badura) is very efficient (i.e., puts the vast majority of light into the 1st spectral order), but with 120mm of aperture you probably won't be able to go much deeper than about the 4th magnitude. There's a good review of the Rainbow Optics unit at:
Start by observing some nice, bright A-type stars like Altair or Vega, which should clearly show the Balmer absorption lines for hydrogen. You could then try observing a cool dwarf star like Antares, which should show absorption lines created not by atoms, but by molecules like TiO.
Spectroscopy can be very addictive! I started with (and still enjoy) the Rainbow Optics model but am looking forward to acquiring a more sophisticated slit spectrograph from Shelyak. For a good overview of amateur spectroscopy and its applications, you could view the recent AAVSO spectroscopy seminar online at: