Very common to pay with debit cards in Belgium. Some supermarkets don't accept credit cards- check first. The speed limit is 120km - but everyone is totally mad on the motorway so I would advise taking it steady. The German autobahns generally have NO speed limit. Where there is it will be signed.
German autobahns do have a defact speed limit. You are allowed to drive as fast as you like so long as you are driving safely. Anything over 130km/h is normally considered unsafe.
Speed limits when you're towing a trailer can be really daft in some countries in Europe. I live in Sweden and frequently drive to the UK via Denmark, Germany, Holland, Belgium and France with a trailer so I can fill up with Cadbury's, pot noodles and bisto.
Anyways, the trailer speed in Germany is 80km/h. See German tourist board website: http://www.germany-tourism.co.uk/EGB/practical_information/driving_in_germany_9847.htm
Belgium doesn't have a seperate speed limit, it's the same as cars without 120km/h.
As you enter the countries you will find there are huge signs reminding you of the changes in speed limit.
Also worth noting, some of the motorways only allow trailers in the slow lane, or restrict overtaking to certain times. There are fairly easy to understand signs up whenever this applies, so keep your eyes peeled.
Also don't forget that in Belgium it is compulsory to have a reflective jacket in the car and a warning triangle. You are not allowed to get out of the car on a hard shoulder without a reflective jacket. We have those little fold up reflective vests for everyone in the glove compartment. They also strongly recommend a first aid kit and fire extinguisher, which are compulsory for Belgian cars. Germany also strongly recommend all of the above plus a set of replacement bulbs as they are also compulsory for German cars.