Travel advice: we recommend that you bring your warm clothes even if the weather forecast notes the sunny days, as the weather in these regions is unpredictable and extreme and it changes from sunny/windy/rainy in almost seconds. Also highly recommendable is the waterproof jacket, as well as walking shoes, sandals to be ready for the weather surprises. And absolutely, you would need insect repellents, sunscreen lotions and basic medicine for diarrhea, headache, cold, insect bites, bandages and any medicine of your own prescription.
We would also suggest that you bring a large bag (backpack or duffel bag-suitcase might not be comfy to carry if your trip includes some hours of horse riding) to be left at your lodging place (hotel or ger) and a smaller bag (daypack, shoulder bag, fanny bag etc.) to carry your most valuable and frequently needed items in.
To get more advice you may refer to www.mongolia-travel-advice.com