Ok there is ALOT of information missing here to even give you a way out there estimate...
1. What are the solar resources of your location and site.
2. Any shade at all?
3. What fluid do you plan to use?
4. What is the coldest temp you will have to deal with?
5. Is the dairy tank rated for the VERY hot water that will be produced. Solar hot water systems can produce water so how that alot of traditional water tanks are not rated for it.
6. What sort of insulation do you plan to use for the tank and piping?
7. What is your rate of consumption of water per day and is there a peak demand period like in the morning?
Do you have a one line or sketch of your system?
Alot of times buying a premade system offers MUCH better results than trying to make it work with random parts. Please PLEASE read this article to get an idea of scope. And refer to the 2nd link to checkout alot more information on solar hot water heating.
It will require alot of calculations to determin the equipment you will need.