Hi @barton welcome to the farmOS community! And thanks for all the feedback and questions.
I’m interested in setting up a standalone server here on the property, and tying it together with an intranet. This app will make the Intranet unnecessary for a lot of things. Is it possible to set this up on a raspberry Pi?
Yes! We have had folks report successful installs on Raspberry Pis, and a little while back I successfully set up farmOS in Docker on a Pi. A warning though: Pis are not very fast - and serving Drupal page requests can be a bit slow.
See: Run on Rasperry Pi via Docker · Issue #58 · farmOS/farmOS · GitHub
Also, regarding offline hosting, see my recent comments here: Learning and background knowledge - #3 by mstenta
I know it’s possible to do it with harvests, inputs, and movements, but a buy and sell button that allows for simple transaction recording would be very useful.
All great ideas! And I have been laying the groundwork for exactly those kinds of features to be built in the future. It would be great if you could add a few specific “Feature Requests” to GitHub that summarize each granular feature idea individually. Then we can start to work them into the roadmaps. Issues · farmOS/farmOS · GitHub
the simpler it gets, the more I like it.
Agreed! And that’s the continual process of farmOS development. Make it work, then make it simple. 
The Field Modules initiative is particularly exciting in this regard: Field Modules · Issue #217 · farmOS/field-kit · GitHub
One of the things I’d like to do is to be able to import my permaculture design layers as an overlay on the map section.
What format are these layers in now? Digital? Hand drawn? I feel like this could be a whole separate forum thread on using farmOS for Permaculture! Want to start that conversation? I’ve thought about it some, so I can add my ideas.