If you are on a tight budget, one of the Chinese refractometers sold on ebay for about $100 will work.Just get a return privilege and test out its accuracy on a piece of quartz, because not all of them are calibrated correctly.If the liquid does not come with it., the 1.8 liquid from ebay is a little less toxic than the 1.81 stuff and about as good.
For other instruments, I'd first suggest getting a copy of Hanneman's book, Affordabe Gemology, as it will teach you more than most other books and allow you to access some really affordable solutions.You can buy a cheap dichroscope and a flashlight polariscope from ebay, too. You may want a small diffraction grating spectroscope. Hanneman's book will discuss a strainless sphere and quartz wedge substitute which will allow you to determine optic sign and character with the polariscope. A cheap digital scale will allow you to determine SG. That is pretty much all you need, although it is fun and useful to have a set of Hanneman's filters for quick ID in parcels. If you had more to spend you could get a small portable UV light like the Raytech versalum, although I'm not sure it is made any more. Another worthwhile investment is a standard magnet for magnetic susceptibility tests (look on line for the site that discusses this.
So refractometer, $100. H's book, $50. Polariscope, $30. Dichroscope, $30. Magnet, $10. Not sure about other prices, but you can look them up on ebay.