I think you can get pre-paid cards with or without phones in some of the larger supermarkets, the HEMA (is one of the cheapest), most larger electronic stores and various specialised phone stores. You can probably already get them at the airport right away with a 10 euro credit.
Your biggest worry could be if you have to pay for staying the night. It's hard to find a cheap place for that, and on short term there may be little room as well.
If you have to travel somewhere by train on your own, the tickets start at 1 euro for really short travel and max out at 50 euro and must be bought before you get on the train. If you don't get a ticket arranged for you along with your flight, you may want to check that out in advance. For traveling inside a city, you can walk almost anywhere in most cities and there are maps around usually. Taxis are insanely expensive, so if you do want to travel inside a city, use public transport, and buy a ticket/pass for that in advance as well (cheaper), though there you can sometimes still buy one on the bus or metro, and that's not insanely expensive (though still easily 3-5 euros), but a cab almost starts at 15 and ramps up fast).
Internet cafes aren't that common over here, if you need access to a computer, best bet may be a library.
I'll PM you my phonenr so you have at least one in case of emergency.
Between having a Master Card and 100 euros, you should be fine though, and yeah, informing the bank in advance of your travel could be smart.