Precautions to be taken for - 20 degree C

Easy
Wear layers of clothing in order to trap warm air around you.
Make sure your outer layer is a waterproof coat.

Thermal under-trousers

Waterproof gloves are essential.

Your head must be covered with something your coat hood will go over easily.

Big boots with two pairs of socks, one lightweight next to your skin, the other thermal and thick. The boots must be waterproof.

I agree with the post from Fred.

I worked in Mongolia for 2 winters. There are many good shops in UB  that specialize in good clothing. Expensive but necessary.

Make sure your ears are covered, and a face covering is handy when things get windy,
These things also get used as a head covering - They're very effective and keep your ears warm as well as the top of your head.

Can I have contact no’s of few Indians for knowing about 1. Availability of mode of commute from airport to hotel , 2. Hotel to office , 3. How to get a good rental flat with two bed fully furnished, 4. Do we get all kind Indian food stuff ( raw for cooking in home ) , 5 what is the cost of the above flat and how is monthly expenditure... etc .
If I know few Indians I can call them .

There are Indian in Mongolia.

I suggest you go the "Community" at the top of this page, click on "members" and check the list out.

Check out the housing section top of this page to get an idea of rentals.

For living costs www.numbeo.com