Wheelchair

If you medically need one then any German health insurance would probably provide one. One would have to find a doctor saying it is truly needed and they would help you with the procedures to get it. I can't answer if there is some kind of small co-pay or if it is completely free. The thing is that people often have varying degrees of handicap or injury – and what they get will depend on their exact situation. What can be difficult is if one has a condition that it is not deemed necessary but still recommendable or desired. The question is then if the insurance will pay or possibly how much. And often such medical devises are not simply given and then the property of the person. They are the property of the insurance company and need to be given back eventually.

And even if you get a wheelchair paid for by your health insurance, it will be the most basic, simplest and cheapest model. Anything higher you want, you'd have to pay the difference yourself.
An example, although not a wheelchair: My 96-years old grand-aunt recently needed a walking frame after recovering from a broken leg. The insurance paid €80 - and she selected a better model costing €600 - with "luxury" features I consider essential, like handbreaks and a built-in basket.

Notharry: Your profile claims that you are from the Vatican. If that is true, you should know how to approach the charity sections of your church for such issues. (But if it's not true, please correct your profile!)

Thank you Tom & Beppi.

Can either of you assist me locate an engineer who can source the smallest lightweight petrol-engine to be used to drive a wheel that will push my wheel chair, total load 120kg up a 20% gradient.

Such things are not available in Germany.
All motorized wheel chairs are electric (never petrol!) and for regulatory reasons you can only get wheelchair and drive as a set from the same maker.