The problem with ETFs for your case is that you will probably want to add a little money every month (a practice that is called "dollar cost averaging"). That would mean buying ETFs frequently, which would produce transaction costs that you can avoid if you operate with ordinary no-load mutual funds through an account in a company such as Vanguard, Fidelity or T. Rowe Price. Whatever account you use, check its tax stance: you may prefer this investment to be in a tax-advantaged account such as a 401(k) if you are a U.S. citizen. Also check if there are extra expenses for the mutual-fund accounts, such as low-balance fees or extra charges for non-U.S. citizens. If dollar-cost averaging is managed well, an ETF portfolio in an ordinary brokerage account can be less costly for certain cases.
Be it through ETFs or mutual funds, you need to build a diversified portfolio. To start, I recommend allocating most of the capital, if not all, in company stock, because your investment horizon is long term. If the market goes down, there is enough time for it to recover. As your planned retirement approaches, shift the allocation towards less-volatile instruments such as bonds. Real-estate and commodities don't look too good and I don't think the amount invested is high enough to justify diversifying through those alternative asset classes.
Regarding country allocation, you may want to more-or-less reproduce the world economy's capitalization but with a bias towards your own country or region (as I presume it is where you will be spending the savings). The reason for this bias has to do with your future expenses being related to the performance of your country's markets. If you are living in Germany and your investments are mostly U.S. assets, a crisis in the U.S. would make your savings too low in comparison to your expenses, which wouldn't be the case for U.S. citizens as their expenses would also go down (measured in euros).
You may also want to allocate a portion on emerging markets, which offer potentially-higher returns (but higher risks too, I'm afraid) and small/medium capitalization companies. If you are a U.S. citizen, the following allocation looks adequate (please receive this opinion with caution, I'm not a certified advisor):
* 25% in a S&P500 index (U.S. large cap), such as VFINX (mutual fund) or IVV (ETF).
* 20% in small cap U.S., with TRSSX or IWM for example.
* 20% in developed-world ex-US large cap index, such as VFWIX or VEU.
* 15% in developed-world ex-US small cap, with VINEX or GWX for example.
* 20% in emerging markets, with VEIEX or VWO for example.
The other fund families have similar offers, always look for no-load funds with small annual expenses. You may want to consider broader funds, such as Wilshire-5000 indexed which include U.S. large, mid and small cap, if you need to keep the number of funds very low to lessen costs (transaction ones if you invest through ETFs for example), but make sure that higher fund fees don't cancel that advantage.
Gradually gear the allocation towards bonds, to end with about a 40% stock and 60% bond distribution at the time of retirement. By the way, why do you want to retire at 40? You will be so young! Anyway, if you change your mind by then, I guess having some nice savings won't hurt.
Apart from augmenting the bond participation, you should regularly re-balance the portfolio, which means returning it to a planned distribution, as it will drift away because of differences in the price movements of each security. For example, if U.S. stock falls and non-U.S. stock rises, you will have too little of the first in relation to the latter. So either buy only those funds that have too-low an allocation when you invest your monthly savings, or sell from some funds and buy from others every 18 months or so, to return to an adequate allocation.
A remaining issue is knowing how much you need to save each month to accomplish your goals. I think there are some free tools on the Web for calculating that (if not, I should program one for my blog :) ). Bear in mind that the portfolio may return an average of a 7% annually net of inflation (don't forget to consider the taxes you might have to pay on that).
I doubt that you will be able to retire after only 18 years of savings or less, but investing and planning in advance are always good ideas. If your plan is not realizable, better to know it as soon as possible, and you may find that with a few tweaks it can become so.
Hope it helps.