Oh forgot about the losi RTR, for starting off from scratch i personally think it is the best car to choose if you are on a budget. Getting a new car with shell and electronics is a good deal for $400.
Yeah racing a simple car like a TT-01 has its advantages in making things simple however it cant keep up in 540 due to the fact that you cant gear it properly unless you start throwing any gear in and dont mind replacing those gears more often as they get crunched. I started off with a couple of TB02s which had the same problem and its depressing racing and knowing you cant keep up with the rest of the field due to the car.
If you are willing to wait and do lots of research then i personally would buy second hand equipment or a Cyclone S/XRay T2R/TA-05 with most of the rest of the stuff second hand. However, there is a lot of stuff which you need to consider so you'll probably just get sick of trying to find lots of second hand equipment second hand in the end. Further to this buying lots of second hand equipment will be time consuming and frustrating as youll just be sitting there waiting to find everything you need and wanting to go to the track and race.
With regards to spares are hop ups, you should have much of a problem getting them in Australia for the Cyclone S. The TA-05 is also pretty good for spares however most people hop up their TA-05s with cheaper 3racing parts which are normally purchased overseas.
TT-01 spares should be good but dont even consider bothering to hop it up. It is just a waste of money. You should get a set of bearings for the TT-01 and probably not much else.
I think the HPI Sprint is a pretty low level car, if you are looking at a RTR kit i would definitely choose the losi over it.