I would suggest you do a test by scanning a few of the newer prints and some of the older prints on any available scanner. See how much dust is on the existing pictures.

When looking at a print physically, a little dust is not distracting. That may change when you digitize it. So you might want to test what condition they are in by testing some of the older and some of the newer ones - maybe some in transparent plastic sleeves, some scattered about and more dusty.

Don't have much advice on what to do if they are dusty, just aware even a little dust can look off-place depending on what is desired.

I did the analysis as well, calculated everything and ended up paying 500 bucks for an external service. Everything else would have cost roughly the same (with buying a good and used slide scanner and selling it afterwards) plus the countless hours for loading and unloading the scanner and retouching, etc. Just use an external service - if he doesn't trust "some online service", I found a lot of small, family-owned businesses in drivable range to go to. They're usually more expensive, but you know where to knock if something goes wrong.

My dad was going to do this himself but it is a massive job. He ultimately sent them to a company and received them back on DVD.

My family even has reel to reel from the 50s or 60s. The problem though is everyone in them has passed on. I am never going to watch video of my great uncle fishing in 1960. Then he is basically a stranger to the next generation.

All this is quite a bit of work for things that will most likely never get watched.

Try DSLR scanning.