You need to know what a disk partition is and know the makes and models of your sound and network cards. Most video cards are detected and easily setup. Winmodems are not supported but most internal modems and external modems are supported. A variety of printers is supported. Intel i810 video is supported.
Software changes all the time.
Internet upgrades are available on a continual basis. Some expertise is required in selecting packages using the internet upgrade as certain new packages may be unstable. Yes, you can. However, installing a Debian package ensures that the software is properly configured and contains all the components needed to run successfully on your system. Maybe two or three.