IP assigning takes place like you say I think - each linode is assigned a static IP by the DHCP server, and will never change.
Bandwidth is measured as a total of both incoming and outgoing traffic (not sure what you mean by X Gb/M).
I read in another thread that caker is setting up a Debian cache locally which would not detract from the bandwidth usage, but I'm not sure how far through that is. Otherwise, apt will work happily with any standard site.
I'm sure someone else will chip in with some more details answers if necessary
