2 Warren Buffett has said that his children's inheritance will be placed in an S&P 500 index fund upon his death

If it's good enough for him, then it's good enough for you
Here's why the S&P 500 is such a great investment:

This depends on where you buy but most funds that are passively managed can go as low as 0.01%
Fidelity even offers a ZERO expense ratio fund. ($FNILX)
That's right. ZERO!
Competition among companies is driving costs down and the investor wins.
Since 1957, the S&P has returned a historic annualized average return of 10.5%.
This is despite all the Black Swan events and bear markets that have taken place since then.
If you have a long time horizon, then you can expect to do well investing in an S&P 500 index fund.