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@flyingcircus, you can find the flight manual in German and Spanish here: https://www.forandoavela.es/d/146-manual-de-vuelo-del-taifun-17e-aleman-y-espanol

In the USA a non-type certified aircraft would be registered in the Experimental-Exhibition category. It could also likely be registered in the Restricted category, but there's not any advantage to doing so. A Taifun is considered a powered glider under FAA regs There are a number of maintenance items you can legally do as owner without the aircraft being registered Experimental, such as oil changes, tire changes, brake work, etc. As far as doing a repair it would require the signature of either an Airframe and Powerplant mechanic (A&P) or, for more complicated repairs, an Inspector Authorization signature (IA). In order to have full maintenance and inspection privileges you would need to build the airplane and then apply for a Repairman's certificate for that particular aircraft. The Repairman's Certificate allows the builder to perform all maintenance and inspections on that particular aircraft, but not others of the same design. Can't help with 2 & 3.