According to this page, the protein and calcium contents in cow and buffalo milk are as below:
Comparison between 244 ml of both types of milk
Cow 8 21% of daily value
Buffalo 9 32% of daily value
Now that, you will be able to drink 2-3 litres of milk per day, you will easily meet the protein and calcium requirements from milk alone.
According to this site
Milk contains the water soluble vitamins thiamin (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B3), pantothenic acid (vitamin B5), vitamin B6 (pyridoxine), vitamin B12 (cobalamin), vitamin C, and folate. Milk is a good source of thiamin, riboflavin and vitamin B12 . Milk contains small amounts of niacin, pantothenic acid, vitamin B6, vitamin C, and folate and is not considered a major source of these vitamins in the diet.
Milk contains the fat soluble vitamins A, D, E, and K.
Milk is a good source of calcium, magnesium, phosphorus, potassium, selenium, and zinc.
So, milk alone meets protein, calcium, and to some extent all vitamins and minerals. An alternate would be curd for days you don't want to drink milk.
Lemons are high in vitamin C, folate, potassium, flavonoids and compounds called limonins. [...] a quarter cup of lemon juice contains 31 percent of the recommended daily intake of vitamin C and 3 percent of folate and 2 percent of potassium.
Fruits, like bananas (ripe and raw) are abundant in Potassium.
Omega-3 fatty acids can be obtained from Chia seeds, walnuts, etc as highlighted in this page. Chia seeds are cheap, but I wouldn't consider it tasty. I prefer walnuts or almonds or cashew nuts.
I wouldn't really buy any supplements as in the linked webpage of the question if I am drinking 2 litres of milk, lentils, lemon (or Indian gooseberry), walnuts (or groundnuts) and any other fruits or vegetables on a daily basis.
There is a question here already, the answers of which have addressed specifically about patrticular nutrients. See this below:
What are the main nutrient deficiency concerns for vegans?
Please, don't buy any supplement without talking to your doctor. These are small things, but we really should be careful about.