Ok i found a complete new underfloor pan for less than 1k $. A brandnew replacement frame costs around 2k $ but it looks good that i can get one from a rustfree donator car for 600 $.
Since i might also need a new trunk floor pan and eventually 1 or 2 other bodypanels, i estimate the costs for all required parts around 3k $, including freight to Europe.
Now the question is how much a professional bodyshop will charge me to replace these parts. If these labour costs are reasonable, i can finally see a light at the end of the tunnel for this restoration.