Can you really get a functioning single prop with new or decent functionally usable used parts that is completely flyable and safe enough for regulations for under 100k?

In the US there is a category called "light sport aircraft" where there are some examples under $100k and many under $150k.

https://generalaviationnews.com/2015/10/12/four-lsa-under-96...

Those are also prices for a new aircraft, used go lower.

Enough economies of scale to make this kind of business pretty much unviable? Maybe. I’m pretty sure you can still buy car kits from car kit companies that are more expensive than a good Honda because hobbyists enjoy building cars from “scratch”. It does decrease the market value of the proposition sitgnificantly though if you are only focused on hobbyists.

$100k isn't particularly cheap for a DIY kit plane. Planes are cheap, it's everything else that's expensive.