1. Yes but you will need Office loaded on the new PC. Old Office documents can be opened with newer versions.
2. Depends on the make and model. Some manufacturers stopped making drivers for XP. Post your make and models and we can tell you. Or check the manufacturers sites.
3. Nope. Might even appear faster as the new PC will render (construct) the pages faster.
Laptops works exactly the same as a desktop for broadband.
If you are backing up to floppy you'll need to make sure you specify that you want a floppy drive on a new Dell as they don't come with one. or buy an external USB Floppy drive. Probably cheaper and easier to get Dell to fit on though.