Given the fairly rigid structure of your intention and the vehicle you will be doing it from, coupled with your intended long-term strategy I would recommend a combination approach.
Invest a portion in exchange traded funds (ETF), like Satrix40, etc.
And then you can also use a carefully selected group of unit trusts to give you the proper required exposure to overseas markets.
By selecting the correct unit trusts with proper risk weighting and diversification, you will free yourself from having to manage your investments hands-on as it were.
When it comes to unit trusts, you select the proper fund manager with a consistent good record or investment house.
Good ones to mention here are FOORD, Coronation (Strategic Income), Allan Gray (Equity Fund)., Nedgroup's Rainmaker Fund.
Each fund manager of a unit trust has a mandate for the fund that highlights the risk weighting, diversification, underlying commodities, asset classes, etc.
Your monthly transfer portion will typically go into a Money Market Fund (Parking), until you transfer it into a particular investment.
You could actively manage that process and select which funds you want to invest in, or have someone do that for you.
Glacier by Sanlam is a good way to do this, self manage that is.
Or approach PSG, many guys ply their trade.
What is key in this game is to manage your costs of transacting. Liberty has recently launched a product that they will not charge you any fees until they have made you a certain percentage return.