There are basically 2 kinds of scanners that can handle developed (processed) film. You have flat bed scanners and small desk top box looking things, like a loaf of bread and these are dedicated film scanners. The word - dedicated - is implied because that is all they will do, film or slides, while a flat bed scanner can do other things as well.
Flat bed scanners that have the built in equipment and ability to scan film are cheaper. Some flat bed scanners can do film up to 4x5 and even 8x10. A dedicated film scanner is this size would be VERY expensive.
Quality.. The quality is getting closer and closer between the two and a Epson flat bed scanner's quality with film ability is way over and above what was available 10 years ago. My Epson is better than my Minolta film scanner. However the Minolta IS almost 10 years old. Epson scanners that can do film start around $100 and go up to $700 +/-.
Look for true optical resolution. Also look for a high D-Max. A number like 3.2 to 3.4 is good, but get the highest you can afford for this is the ability to record and look into the high's and low's of the film (the light and dark areas) and get detail.
As said, the film HAS to be developed first. Any place can develop the film. Wal Green's. Wal Mart. CVS.. it don't matter, just ask for develop only. NO prints. Scanning the film will NOT hurt it and being exposed to light, after developing, will not hurt the film either. 99% of the stuff that makes up film is the plastic base that carries the emulsion that the image is in. If the film is coiled up and left for a period of time a memory sets in and it wants to stay in a coil. However, if cut into strips and slipped into the pages, time and some weight can make them all lay flat again, and they will stay this way.
Film scanners can change any film camera into a 'digital camera' thru their ability to scan the film that went thru the camera. By doing it this way you can have a "full frame digital" and because the scanners resoultion will often be higher than any pro digital 35mm type camera, you now get better images, in a way, than the digital camera could.
Something to think about..
Bob - Tucson