I am about to apply for residency as a self-employed worker(I am a freelance photographer). Is it advisable to have a list of clients as part of my business plan, and can they be German firms?
I am also unclear as to whether or not a certificate of good health is required, or if evidence of my health insurance addresses this issue.
Thanks!

they will send your application to employment office first and ask whether german market really needs you badly. as you do not employ any natives the answer may be "no" => then you don't get residence permit regardless of your business plan and evidence of health insurance

accordingly to business plan: immigration office has no interest in business plan or list of clients whatsoever, they want to have proof that your business is not just a 1-month or 1-year-business, that it will survive and prosper. moreover, they always want to be able to say that they are in no way responsible if something goes wrong with your business. therefore they usually require a kind of expert opinion from IHK (Industrie- und Handelskammer) that gives it to you based on your business plan. if you want to know what your local IHK wants to see in business plan, just call them and they will give you required information

immigration office may require a letter from your health insurance company as well where it will be written that you are insured by them. but again, they can issue you a residence permit without asking for that letter.