I've scanned a few thousand slides and with just a few hundred left I dread finishing the task, it s PITA if you want decent results. You are essentially photographing the scene again. You can use the same settings for a similar series of slides, but when things change you must adjust. You also need clean any dust off and depending on how much you spend for the scanner and the resolution you chose it can take up to three minutes per slide.
If you buy a scanner get the best one you can afford and you are going to need the bundled software and drivers so beware of used equipment. I bought a Plustek OpticFilm scanner, it works pretty good, but it took a lot of experimenting to get it operating smoothly.
If you have a service that you can afford, have them do a sample of the various types of slides you have and see if they do a decent job.