As @farnsy already said, you need to make sure that the small profit you can get from very small investments is not completely eaten up by transaction fees. I'd like to add a 2nd very similar point: taxation can mean quite complex burocracy that creates basically the same hassle for 100 € or 10000 € investments. A practical trade-off may be to save your monthly budget for investments and instead of monthly investments on the order of magnitude of 50 - 100 €, go for one transaction per year in the order of magnitude of 1000 €.
That being said, personally I deliberately decided to start with rather small amounts (though not as small as 100 €, but maybe only a single trade in the first year - particularly as you are going for an ETF that inherently provides already some diversification) and to slowly add (burocratic) complexity while researching and learning about investing. Small amounts in order to see whether I correctly understood the not always very clear descriptions of fees and tax consequences, accepting relatively large transaction fees in order to be sure that mistaken conception of the "side-fees" wouldn't affect large amounts of money... And while there are lots of online brokerage simulations around, they usually do not simulate the complexity of fees and tax declarations.
As you mention Germany, look up the new (2018) taxation rules for fonds. According to boerse-online.de they are supposed to be mostly handled by your bank, but I'd nevertheless recommend to look up what they mean.
The new rules are among other things supposed/meant to put foreign fonds on the same overall taxation with German fonds, but I guess that doesn't necessarily mean that the amount of burocratic work is the same (e.g. for some foreign shares, both the foreign country of origin and Germany will automatically deduct full taxes and you can then later on claim back foreign taxes due to a double-taxation treaty. Which is a non-neglible amount of work and your bank/broker may charge additional fees for the additional tax proofs).
So for the beginning, I'd recommend staying with an inner-German product, even if (seemingly) attractive foreign (incl. EU-based) products can be bought quite as easily. And with a German broker: tax-wise the only risk there is that you accidentally pay too much taxes if you don't realize how to get double-taxation refunding. OTOH, with foreign based accounts it's up to you to declare and pay the taxes correctly. I.e., simple honest mistakes can get yourself into tax evasion - with much more serious consequences than the loss of few € due to having paid some taxes that were not due.