by OP:
3. At some point in time, I would like to build my own planes, from kits and maybe, my own design. Where does one find out what restrictions there or, regarding this.
By Able:
There is a de facto restriction on weight of 25 Kg, set by an international organization of commercial aircraft operators (ICAO), and agreed to by the US, represented by FAA. FAA has not yet felt the need to enforce it, leaving that for the time being with organizations like AMA.
Seems we just went over this, to well beyond a gray area, in the thread [link=http://www.rcuniverse.com/forum/m_4488019/tm.htm]FAA regulating model airplanes [/link] .
There we had a lovely little chat about AC91-57 from the Fed, the FARs of GA, and the SELF IMPOSED ama 55lb limit. Any concerns of questions about the weight & altitude limits should first read the 2 or 3 pages of that thread. As that thread left off, there is no actual law forbidding planes over 55lb (local ordinances ofcourse may vary), but folks should feel free to limit themselves for Niceness and Internationalismness. However, the ama regularly grants exemptions to that limit, anyway.
Build whatever make you happy, its a Hobby- enjoy it.