Start here for Aussie shopping
http://www.*********.com.au/store/index.php?cPath=68

If your budget is $100 you are not going to get a lot.
Do you have any RC gear. Battery packs, Radio gear, charger etc.
I have done a lot of reading and I would say that upgrading a NQD boat is a long and expensive journey.
The radio set they have is very primative......replace that
The battery and charger is going to be of questionable quality............replace them
The motors are cheaply made 380's...............cook them before you replace them
The hull is heavy plastic..........will need work to get it to go fast.
You follow so far ????
I have been down this path of reasearch and will not do a NQD boat myself.
I have hotted up a couple of ABS hulls and am very happy so far.

Have another look on e-bay and start looking for second hand good boats or basically anything that does not have the words "vector thrust" or "NQD"

If in doubt just type "nqd" into the search box here and have a read. I have.