Telstra isn't rolling out the NBN. NBNCo is, so they determine what gets rolled out.
As far as paying for faster speeds. Get connected on the standard speed, then have a look at the maximum connection rate that your modem reports that it can connect at. Then make the decision as to what boost you get applied.
FTTN is VDSL technology and suffers from the same distance related drop (although much higher speeds) as ADSL. If you are at the end of a run, then you may not be able to achieve 100Mbps and it would be silly to pay for it.