Again, just to re-iterate, voltages and other intnternal specs will NOT be any different. The hardware in PC's is pretty much universal no matter where you buy it, the only difference is the language the drivers are in.
If you buy a machine in the UK, then upgrade with parts from France, Germany and the US, it'll all work together.
As for the TV output, yes, you can normally select either PAL/NTSC. Some even auto-detect. So, no need to worry, just buy the damn parts already