Yes, there is a charge for changing speeds. Yes, but the DSL circuit would need to be reconfigured for new speed. Although there may be a few of the early subscribers who started out with ADSL who have bridged connections. The bridging will be phased out eventually and replaced with a routed mode. You get one static IP address.
Additional addresses are optional.
In many cases a user will not need them. Assigned subnets are given only on a case by case basis. A special request in writing should be submitted. Speak to your APK Net sales contact. A DHCP server is a router that uses the DHCP protocol to assign and manage IP addressing.