Hey mate, i race 125's. How old are you? Are you looking at doing a club championship, state championship or national champs?
You can spend any amount on a 125. Blake leigh smiths engines cost $50,000+ each, with hand made crank cases from belgium. But it is still a class where the right jetting and gearing can get you the win, as Old Man Galvin proved at Symmons plains. If you are planning on doing a club or even state title you are best to buy a well-sorted older bike.
Any Honda rs125 after 1996 is quick (before 96 they weren't the most sophisticated). TZ's are good if you can find one. All models 94- come with three-way adjustable suspension and power jet carbs. 94-97 they were short stroke engines running power valves. After 97 they are all non-pv square motors. If you can find a 97 or later TZ with the Kit power valve setup this is the way to go. If not, you can find clean RS hondas with spares for 5-6K.
It all depends on what you have ridden before. These bikes are totally uncompromised, and have rediculous corner speeds (the fastest of any class). Point being, you will probably be slower on a $50,000 Aprilia rsw125 than a 95 tz125 at the start because they are harder to ride. Advice being, get on
The Post Classic Racing Association of NSW - PostClassicRacing.com.au and get into the discussion section where there are classifieds. Keep an eye out at
www.formula-extreme.com.au. You will start seeing them come up for sale at the end of the year after this season has finished and people change classes