I would suggest a Melodie Susie for a good, budget-friendly starter. One with the LCD showing how many thousand RPMs it's going.
Then, when you have more expendable income, invest in either a Young Nails e-file or a Kupa. Kupa is great because they service their units, but lack an LCD screen. Young Nails has a screen, but no service department for fixes/cleaning.
Also look into Kiara Sky. I hear lots of great things from them.