I recently bought a second hand 3.8m inflatable for ~$800 and a second hand 25hp in reasonable condition for ~$1000.
They are a great diving platform. So stable you can have three people sitting on one edge and barely cause it to lean. Also able to safely take on a bit more water than a regular boat so good for getting through surf, not that I'd recommend pushing it..
You'll need a licence but the test is pretty cruisey. Can get the vic boating regulations from vic roads website or a hard copy for free from any vicroads office.
Launching from most beaches is generally possible (weather/swell dependant) but they are pretty heavy to carry and take a while to set up so maybe not as convenient as you hope.
If you're more motivated that me I'm sure you'll have no dramas