It's a field where I don't really have much credibility since I'm an interior designer, not an expert in car purchases. But I have a friend who got into something similar last year and it really stuck with me, all the hassles she went through. First, you absolutely have to check that the vehicle can legally drive in Belgium - emission standards and safety equipment vary depending on the country. Get the vehicle's paperwork checked (German registration certificate), make sure the title of ownership is clear and there are no liens or mortgages on it. You also need to verify the accident history through German databases, because you don't want to find out too late that the Golf was a repaired total loss. Where my friend got caught out was she didn't think about insurance before delivery - you have to have coverage before you drive, even to cross the border.
The other important thing is potential taxes and customs fees. Technically if it's a used car imported from Germany to Belgium, you have to declare it and pay VAT according to Belgian rules. You also have to check that the German technical inspection (TÜV) is up to date, but know that Belgium will ask you for a new technical inspection over here. Honestly, getting the sales deed across borders is complicated - I really advise you not to sign anything before consulting a notary or at least reading the Belgian legal conditions for importation. Watch out for typical classified ads scams too: never send money before seeing the car in person, ask for an independent inspection if you can't go yourself. It's an investment after all, better to be careful.