Having recently done a bit of research on this, I'd recommend Drive FTI, not only for the best price, but also because they cover the deductible (CDW) 100% in case of an accident. You can't book directly with DriveFTI but need to go through and agent, e.g.
ADAC (if you are a member), or TT's good friends at
Just Travel. Note that the ADAC is offering special deals on Winter Ti(y)res on their rental deals at the moment. I'm sure Just Travel, who conveniently speak English will also be able to ensure you get the extras you need. A quick search of their site offers a 7-seater Espace for less than €400 (fully inclusive) for a weeks hire
One word of warning with the bigger rental companies - they may only offer Winter Ti(y)res etc., at substantial (daily) additional cost, and if you don't take them will make you sign a disclaimer that absolves them from responsibility of there is a winter accident. Remember, you are now legally obliged to ensure the car you are driving is up to being driven in the conditions you find yourself. On snow-covered roads, that means winter ti(y)res are a legal requirement...
YL6