Thanks for bringing us back to the topic!
You could try to investigate renting in Hong Kong for week, there's more hidden away in Hong Kong than people think, and it would be safer and more legal.
If however you still have your heart set on China. I think you should buy 2nd hand on the edge of Shenzhen or Zhuhai. Try to just pick up a regular 125cc bike. I don't know about Guangdong province, but where I am 2nd hand prices from 3-4 years old hover at around 1500-2000 (US$250-300) Yuan as long as the bike is in reasonable condition, so you'd lose very little money in resale.
Make sure the bike comes with a big green registration book with up to date insurance, and a small blue motorcycle photo ID card book, with an up to date insurance card in it.