I you spend 400k to buy a house, on a 25-year term with just 4% interest rate, your monthly payment will be 2111 euros and at the end of the repayment you will have paid 233.404 euros in interests alone.
Would you prefer to make mortgage payments of €2k per month for 25 years or pay rent of €2k per month for 50 years? The answer should be obvious.
Also, inflation will gradually reduce the real cost of mortgage repayments and gradually increase the real cost of the corresponding rental payments.
Anyhoo, you originally asked which ETF for €100 per month.
In my opinion, the answer is none.
Unless you like filling in a spreadsheet on a monthly basis and filing tax returns, it’s just not worth the hassle for the expected after-tax return for such small sums. Your time also has a cost, unless you like working for free.
The better approach is -
(a) buy a home that is suitable for your long-term needs;
(b) maximise tax-relieved pension contributions for your age; and
(c) pay down debt, including mortgage debt, ahead of schedule.
However, if you still want to invest your after-tax savings in an ETF on a monthly basis, I would invest in an ETF that tracks a global index like MSCI World.
I have no idea what companies will outperform in the future, so I would stick with the global market consensus.