Expanding on my various comments, buying late is almost always a bad idea, especially for low-cost airlines. The main answer to Flight tickets: buy two weeks before even during holiday seasons? explains how this works and also more or less applies to low-cost airlines (perhaps even more so as very few people will ever change their booking with low-cost airlines as it's almost impossible or at least quite expensive compared to the price of the ticket itself).
Additionally, low-cost airlines are not particularly happy to sell tickets at the airport, an important aspect of their business model is to minimize customer contact and have everything go through their websites. For example, Ryanair is notorious for charging seemingly absurd prices if you forgot to print your boarding pass at home before coming to the airport. If you value flexibility, this is definitely not the right choice.
Consequently, you can compare prices between your multi-destination ticket, regular one-way tickets and low-cost fares now. If low-cost airlines are not cheaper, it's not going to get better shortly before your trip. On the other hand, if you value flexibility, you will have to decide if the price difference is worth it for you. You might also want to consider rail passes (at least for Germany, the Netherlands and the Czech Republic; Greece is not well connected to the European rail network and long-distance trains in France are best booked in advance).