1, 3, 4) There are a number of suppliers of enzymes and the like, which ones are the best depend on the enzyme and what you are trying to do with it. Questions you need to ask yourself or your lab manager/PI: do you want the diagnostic test itself, or just the enzyme involved? Do you want the actual diagnostic enzyme used (very expensive usually) or can you get by with a product that is the same, just not certified for diagnostic tests? What form do you want the enzyme in - dried powder, liquid (i.e solubilized)? How active do you need it? How are you going to store the enzyme (in what form and what temperature)? How long do you want to store it for? What substrate (e.g. alginate) do you want it on? A literature review for your area of research is a good idea here, it will help you know what others have done, how they have done it, and probably get you names of suppliers for some of the things you want.
2) 200 U/mg means that each mg of powder in a bottle/tube contains 200 U of enzyme - the powder may not be all enzyme, or some of the enzyme may not be active, so the molecular mass of the enzyme is not relevant to the calculation of concentration. I suggest that you look at the definition of the term "enzyme unit" relative to the enzymes you are investigating - it is a definition of the activity of the enzyme, nothing to do with the substrate, and often varies depending on the enzyme and the temperature of the reaction.
additional 3) It depends on the enzymes, the substrates, and what you are doing with each enzyme and substrate (storage conditions etc). Some are stable, some are not.
additional 4) It depends on the enzyme and the storage conditions - some might work well after storing at room temp for a day in liquid, whereas others might last a year, or an hour!