If you are in the US you have a lot of regulatory to contend with. The US Milk industry is one of the most heavily regulated. There is a guidance document known as the Pasteurized Milk Ordinance. Latest revision is 2015 with revision 2017 to be published sometime early next year.
At any rate, one of the regulatory stipulations would require you to re-pasteurize the milk regardless if it is already pasteurized or not because you are considered a "processor" and "packager" of milk products. The other stipulation you need to understand is standard of identity. Milk products have a standard of identity.
If you are doing flavored fluid, drinkable milk, then you cannot add preservatives to it.
Also, there are differences in requirements between Federal milk and California milk. I don't know where you are located (if within the US), but if you are in California or plan to distribute to California you will need to understand those differences.
Good luck in developing this on your own. However, I can tell you from my experience people who are in your shoes typically look for someone to co-pack their products for them. I'm not trying to sell you on anything, but our fluid dairy plant only does co-packing and we assist with R&D development through commercialization for new products. If interested you can drop me a line. If not, no worries...I'll help you where I can.