I've had a similar dream for the last ~7 or so years, specifically Guerneville near the main village. Have swung and missed on half a dozen SFH's in the last couple years, the area really took off during COVID and I saw properties going $100k over list. I also considered the land idea and sticking a BOXABL or shipping containers on a plot and building that way.
Couple concerns.
That area, especially near the river, is pretty remote. Building from scratch will likely cost $$$$$ and then add ~20% for overage for materials, shipping, delays, permits, who knows what. Running power and water. It's CA, contractors are never on time or budget, and it is a hard to reach area. Nothing related to building there, will be cheap, IMHO. Could be wrong. But you're going to need to find contractors that probably have to drive out there every day and back, and there's a cost for that too. Hence the modular building idea to cut the time and cost down.
This area, like many, underwent a big STR license ordinance during COVID. Pretty sure there is still a moratorium, or the max limit of licenses has been hit. I stopped my buying search once that happened, as it would be tough to make a rental work in the area as an LTR or MTR at current prices, unless you really found a steal in a prime location. So your note about keeping it and using as an Airbnb is a risky option, depending on what happens with licenses in the future.
The only way to know what a hyper-local cost to build would be, is to interview local builders and GC's who service the area. None of us can answer that for you - could be $300 per SQFT or could be double. I would also call a few agents to get a feel for building, contractors, permits, timelines, etc. You might be sitting on land for a year waiting for permits, soil reports, inspections, etc. etc. before being cleared to build. I would also ask agents if they think a 400-600 sqft unit is worth building for the effort, or if you need at least 1000sqft etc. To go through all the hassle of hiring, permits, trucking stuff out there, for a 600sqft unit seems a bit foolish. I'm honestly not sure this is the right area for a new build / flip idea, because it will be super expensive, and unsure what the profit margin would be. It would need to be a spectacular property most likely, not a tiny home.
All I got. GL! Keeping my Guerneville dream alive, but on the backburner...