So no need to rip the discs as ISOs? Yeah I noticed with 4 GB left to download! Is an easier way to sort of do this?
Nah, you can use the disc as a coaster now if you like

Steam basically deals with all the downloading and patching of games. It automatically patches it when new content is added, and if you decide to uninstall it, or format your hard drive, you can just install Steam again, and TF2 will be in your game list, right click it and install.
At release, there would have been more point to using the DVD to install, as there wouldn't have been much changed, but the game has been out for three years now, and hugely changed in that time, so it'll be faster just to download all the new files rather than install the old ones and have to overwrite them all anyway
As for learning how to play classes, the best way is to just play them. The training mode should give you the very basics (Though I've not actually played it), but if you've played any FPSs before, most of it should be fairly obvious.
http://wiki.teamfortress.com/wiki/Main_Page
The TF2 wiki has tons of info, click on the classes and it'll show you the stats, how best to play them and so on, to help you pick out where to start. Also check out the
Meet the... videos
