The other alternative is to get a LED light box, and maybe a loupe (or a magnifying glass). This would at least allow you to survey the collection. It would have the advantage that you could annotate the slide mount when you've worked out the subject, harder when the slides are going through a carousel.
A problem I found with my New Zealand slides when I came to scan them 40 years later, is that in the course of a few slideshows the slides had got all mixed up. There were even some Tasmania slides amongst the NZ ones. It took me ages to sort them out. In the end I had to use all sorts of clues like the colour of the slide numbers (there were Kodak card mounts; harder with the un-numbered plastic mounts!). The light box would also allow some sorting.
When I started retro-scanning, I did check out external scanning services. There were quite a few available, and the unit price could get quite low for large numbers. However, the price was high enough that I would definitely only want to send the better ones for scanning, and I couldn't think of a reasonable way of making that selection, and getting the returned slides into their proper places later. So I decided to do them myself. It took me a year or so of intermittent effort, but I'm really glad I did.
