And since XXXX10 is not from the US......
1. No, only when crossing national borders and if you want alerting service.
You can use beacons/waypoints, Lo/Lat, and allso geographical names in some countries.
2. You can fly VFR up to FL195 in most countries, given that it is class C, D, E or G airspace. (D is most common in the UK) Class A does not allow VFR (generally, there are exceptions).
Lowest IFR level is Minimum Sector/Track Altitude, published in all IFR area charts, and this has to to with terrain/obstacle clearence.
3. Yes, but you need a special rating.
4. In the Eurocontrol area: All flights in controlled
airpace (except class E) need transponder if the ATC unit is radar equipped. Exceptions are gliders, UL etc wich may operate in special designated glider sectors.
And on another note, please do not apply the information given by our US friends here to real life. The way airspace is classed through the different levels etc is completely
different here on our side of the pond!
CTR and TMA's: Class C or D (some places allso A in fact)
Rest of FIR: Class G from GND to about FL70-Fl120, then class C or D up to FL245-285, then class A to FL410-460 or unlimited. If Class A has an upper limit, it's class G on the top.
Class E airspace is less used over here, but you can find it both in TMA's & FIR's om medium to high levels, and on airways below C/D airspace. (FL50-100)
This very loose, but quite representative for the UK allso I think.