After reading all of the comments, I think:
1/whether your gf is trustworthy or not, because buying real estate, even just down payment, will cost pretty much.
2/ Buildings built by HK companies and buildings in HK will have better quality for sure, but houses in HK will be much more expensive too, is your budget OK to afford house in HK. If your budget is OK, then HK will be a better choice for sure, if not, you can only choose China, especially a city still developing, there will be more space for the house to appreciate.
3/ Check carefully before you sign the contract to buy a house. But there are not as many scam as you think in China. It is the 21st century and you are not stupid, no need to worry about a scam if you have more than 1 friend beside your gf in China. Ask your other Chinese friends for help before you sign it.
4/ And I do agree somebody said that, if you treat it just an investment, dont do it, buy some long term fund in HK will be better. But if you treat it as something that strengthen the bond between you and your gf's family, go for it. Even you cant profit from it, at least you have a house to live in right?
