Zane a few things that will help you decide is some background.
Now we have various ISP's that have their own ADSL equipment in the telephone exchange, they pay a nominal rate to telstra for the lease of the copper line to your socket. The ISP ADSL port is connected to your copper pair at the exchange.
With NBN this changes, NBN becomes the wholesaler and ISP's lease bandwidth off NBN on a node by node basis. The NBN node will be a green box near a pillar roughly within 800m from your house*
Now when you migrate to NBN , the copper from the node to your house is NBN's to maintain. There is NO LINE RENTAL on the copper to your house. Your provider pays the lease to nbn.
When you connect to NBN the telephone exchange(equipment and copper pair to the node) is now redundant. Your new or existing phone number is downloaded into the modem. If you want to retain any hard wired telephones there has to be a simple wiring change inside your house. The incoming line needs to come from the street directly to your modem, then dial tone will come from the phone 1 port from the modem and back to your existing wiring to connect any hard wired telephones. It is possible to have nbn but only a phone or nbn no phone.
So to answer your questions
1) Yes 2) 3 or 4 3) No
Because ISP's lease bandwidth from NBN you have to closely study the fine print of the plan. Someyou will get more usage but lower speed, others less usage but higher speeds. Generally 25/5 is plenty for $70-80 a month you will get a 500GB/unlimited bundle. For instance I have 100Gb plan on 25/5 with all the free calls and pay as you go.
Big advantage of NBN is it is a very stable platform,condition of the copper line is irrelevant and 25/5 speed guaranteed