This is a lovely idea

See if you can get a national newspaper or media company involved as good long term publicity is vital for success

"Do you think I shall just let the normal photographers to take part in the competition and raise money for the disabled?"

Absolutely - with modern cameras disabled people are on an equal footing with the best photographers in the world, provided the have the facility to steady a camera (like a functional camera bracket fitted to the wheel chair etc) - Being disabled myself by Parkinsons being treated as a normal person is very pleasing and I think it is good to integrate rather than segregate.

"Any idea of raising money for them?"

Selling pictures, creating a calender, book etc., may provide additional income (and good publicity will swell sales)

"'What topic would be suitable?" Street photography - Wildlife - Around the Home - Holiday and Travel etc.

Best of luck and if you think I can help in any way just contact me.