We recently bought a piece of land in Germany and are currently going through the process of getting building permits, so I'm somewhat up to date on the bureaucratic aspects of these things in Germany.
20.000 Euro will basically buy you 1ha of agricultural land, if it is considered decent by farmers. Woodland is less expensive, but also often less productive, as are less desireable plots.
BUT: you have no chance at all of getting a building permit (which is needed to install any type of permanent lodging) for agricultural land. In fact, you may even have to get permission to change the land use type (from woodland to field, or vice versa), although this is less complicated.
Building permits are only granted in specially demarcated areas (this needs a vote in the town council and is a very political process) or on plots that can be build on "by right", meaning that the plots around them already feature permanent lodgings. As you can imagine, buildable land comes at a premium in most places as a result. The exception are probably very rural areas (more than 3 hours from the next major population center) that have experienced population loss. This includes some areas in South-Western Germany, but primarily rural parts of East Germany. There you can find cheap "fixer-uppers", although good agricultural land will still be pricey.
I would support those in this thread that have advised to put "people first", especially if you plan on building a community. Moving anywhere with 20+ people will arouse suspicion by the locals and requires detailed knowledge of the laws and regulations. This will be no different in Bulgaria, England, Spain or Belgium. Speaking the local language is probably a basic requirement, as local officials especially in rural areas might not be fluent in English (I know they wouldn't be here).
I would suggest defining a few target areas along criteria like climate, language/culture and the general availability of cheap land. Then go there regularly on holiday or shorter visits, talk to the people and let them tell you what they think of your plans. Find local groups that share your goals. Build a network of people that can help you navigate the process.