1. You will need to run the installer, and use the script for going from v2 to v3 to update your database.
2. the confirm and cart page should be the same as before, although there are not identical. I recommend to copy the eWay code from PS2 and past it in PS3 pages where it applies.
3. yes, but again i recommend to make the changes on the PS3 pages instead of completely overwriting PS3 pages.
4. For the images, you will need to copy the images from Images\Upload to the imagesupload folder in PS3.
5. PSv3 is 4.5 and 5 compatible.