Stock not available at home country nor at their local market - where should I buy it
Theoretically, it shouldn't matter which one you use.  Your return should only depend on the stock returns in SGD and the ATS/SGD exchange rate (Austrian Schillings? is this an question from a textbook?).  Whether you do the purchase "through" EUR or USD shouldn't matter as the fluctuations in either currency "cancel" when you do the two part exchange SGD/XXX then XXX/ATS. Now, in practice, the cost of exchanging currencies might be higher in one currency or the other.  Likely a tiny, tiny amount higher in EUR.  There is some risk as well as you will likely have to exchange the money and then wait a day or two to buy the stock, but the risk should be broadly similar between USD and EUR.