1) HMV and Play have quite a few HDDVD titles starting at about £2.99 each, as I am sure will Ebay, and there will no doubt be warehouses somewhere full of them.
2) HD-DVD discs are region free I believe.
3) Previous generations of LG PC Rom drives could read both formats, I have one, the LG GGC-H20L (£60), but no HD-DVD discs just BluRays.
4) PowerDVD 7 Ultra on PC could play both formats on PC but this was later dropped as of version 8.
5) It is possible to back-up/rip HD-DVD discs to PC as entire disk images or just movie streams, but this is not the forum to discuss such things further, please look elsewhere.
6) Picture quality and audio are 99% the same for both formats, the only real difference being the slightly lower storage capacity of HD-DVD and the fact Dolby True HD is stored in a different way on each format.