A DMG file is a disk image. After downloading it, double-click on the image and it will mount the virtual disk. On that disk is the Logic Pro 6.4.3 installer package. Just run that and it will update what it needs to.
As for finding 10.3, there are probably many places online that sell it. The first place I could think of was
Other World Computing which has it for $48 US.
Cheers!