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.
https://www.fda.gov/...k/ucm513508.pdf
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.
http://milkfacts.inf...of Identity.htm
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.
https://www.cdfa.ca....ry_food_safety/
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.