If ambulances cost in Germany what they cost in the US under a universal health care program, then they would either ration them or charge exorbitant taxes for them.
Yes, medical care availability and quality suffer as the consequence of the European model, and that everybody regards it as "free" (despite having been taxed through the roof) doesn't help. On balance, it's probably better than the US system, but it's not the rose garden a large chunk of HN seems to think.
50% is, as very rough ballpark number, your net salary in Germany. Usually it is more than that. Not all of the other 50% is taxes so. It is social security, unemployment insurance, salary taxes and part health care (usually around 15% with your employer paying the same amout directly as well).
And nobody thinks health care is free, just universal. It might seem free as everybody is used to pay part of his salary every months but health care cost is a recurring topic over here.
Availability for emergencies, and ambulances are for emergencies, is by no means bad. There are medevac helicopters placed in a way to minimize response times. And they are covered by your basic insurance.
Availability can be an issue compared to private insurance due to budget constraints in the public model and the fact that doctors and hospitals have to live. Again, emergency services don't suffer from that. Could overall availability be better? Yes. Is availability overall satisfactory? I would say yes as well (point in case, general doctors in the countryside, there are not enough of those for a variety of reasons).