In my opinion, none of us really knows how much risk we can take until we've lost a lot of money (on paper) in a short period of time.
For the poster above, Toyota is not in U.S. market indexes (as far as I know), neither are many other global companies who happen to be domiciled elsewhere, like SAP, Unilever, Tencent, Alibaba, or Nestle. I think some international diversification is likely to improve expected returns in the future over the long term, but I could be persuaded of behavioral benefits to just plowing into one index fund and not looking at it.
One option might be to use Series I bonds for a bond allocation, especially if you are maxing all other tax advantaged space, could even double as emergency fund after seasoning for a year.