1) yes mode C would be sufficient however there will be limitations, for example Germany will not allow you higher than 5000' AMSL. You could in fact cross Germany and Belgium without any transponder at all, but I think Hungary requires one. No idea about the situation in Austria - I do know they require an ELT there, though.
2) no, the microlight (called ultralight outside the U.K.) license is totally separate from all others. In fact, microlight licenses are nationally administered, you would need to have one national ultralight license, then make sure there is mutual acceptance of them between the authorities of the other countries visited/transited. That last point would not be too critical, I believe. But EASA/ICAO licenses have nothing to do with ultralights.
3) sorry, not a beginning of an answer there
Some afterthoughts:
-) perhaps consider doing part of the trip on a trailer? A lot less doubt about the weather, and it can be as fast, because one can drive at night. My own came from Hungary that way. Ferries would make the last part expensive, though.
-) instead of flying over Austria, consider routing via Czechia: that country is full of small fields with friendly helpful people. Not to say anything negative about Austria, though, only I have never flown there.
-) another point of concern is the overflight permit - in all theory, you would need to pay +/- 90 € to be permitted to enter Belgian airspace. In the real world, things are more relaxed, though.
Last edited by Jan Olieslagers; 25th Jun 2016 at 13:48.