Hi, by chance I’m from the area. I find the radius Mainz / Bingen to Limburg immense! The farther away, the cheaper (property), but then you have the commute to work! You have to like that! Besides, neither in Bingen nor Limburg is it cheap enough that 300k would be enough for everything.
So building yourself with those numbers in the Mainz-Bingen area is unfortunately completely out of the question, and you won’t even get a used semi-detached house without major renovation backlog for that.
A week ago we looked at a semi-detached house here, with towel garden and leased heating system. It took less than 3 days for the realtor to find a buyer at the asking price (350k). We personally wouldn’t have paid that, just because of the heating issue (what people lease nowadays). With additional costs, that would have been 400k and we’re talking about a 20-year-old semi-detached house 40 km southwest of Mainz/Wiesbaden. Honestly, I can’t see how anyone could think to build here for 300k.
The closer you get to Mainz, the more absurd the prices become. Half a million for an old but renovated semi-detached house in Stadecken-Elsheim. Of course, more than 60k additional costs come on top of that. But you won’t have to look for a buyer long for that either. Crazy...
The moral of the story: forget it and rather look toward Hunsrück or the district of Bad Kreuznach, there you might have better chances.