At 500USD a month, if I were you, I'd buy once every quarter - $1500 (US) on a single stock. This will keep your transaction costs manageable @ HSBC (US$ 18 or less depending on their promotions in effect at that time).
Or I'd split it at HK$2000 (or whatever the min is) and get into 2800 every month, if thats in your target investment and lump the rest quarterly.