What is the easiest and lowest-cost way to buy into S&P500 ETF / Hang Seng Index ETF?

You could gain exposure to S&P 500 by purchasing the SPDR S&P 500 ETF (SPY), iShares Core S&P 500 ETF (IVV) or Vanguard S&P 500 ETF (VOO).

As for the Hang Seng Index, you could consider buying the Tracker Fund of Hong Kong (HKEx: 2800), managed by State Street Global Advisors, one of the largest asset managers in the world. The expense ratio of the fund at a very competitive rate of only 0.09% per year.

Another alternative to gain exposure to the Hang Seng Index would be the Lyxor UCITS ETF Hong Kong (SGX: A9B), first listed in Singapore in March 2007 and is denominated is US dollars.