Which GDP per capita of a Chinese citizen would better reflect their affordability- Nominal GDP per capita or PPP GDP per capita of China? My question is would PPP would matter much in case of buying and selling international goods and services?
To me, the method to get PPP value could be crude. If you're only interested in an iPhone, here's what I would do.
Get the price of an iPhone sold in China (in CNY), divided by the nominal GDP per capita for China (in CNY). You'd get (roughly) the fraction of annual income a Chinese person would need to spend on an iPhone. That should give you an indication of whether that's "cheap" or "expensive"
To compare across countries, you'd just do the same and see if that fraction is higher (more expensive) or lower (less expensive).
Keep in mind though that, with existing income inequality, GDP per capita doesn't really tell you about the actual income an average person gets.