It all depends on the weather and what part of the trail you are on regards altitude and temperature. Also your fitness/speed depends how warm your body will be and how much you sweat.
Personally I wouldn't wear down while trekking as it is not very breathable and will end up wet and cold, you are better of with thermal/breathable layers which you can ad/remove and are designed to dry quickly. For sitting around especially in the evenings you will want a single big warm "michelin man" style down jacket. You want to keep weight and hassle to a minimum so just bring the one, and if you do end up too warm you (lucky you!) just unzip it, remove your hat etc.
As to what/where to buy...
If you do walking/trekking a lot I guess you have your own, otherwise buy one online before you leave as it will be cheaper for a genuine one and you will have a better selection.
If you don't do a lot of hiking generally - and you have time - you can buy a lot of cheap imitations in the small shops in Kathmandu. The shops always say they are real but you can tell by the price, stitching, zips etc. They will not have as breathable fabric (though I wouldn't use it while walking anyway) and will have synthetic fill. These will be slightly heavier, less compressible, will not trap as much air (lower "fill power") and will not regain as much loftiness after compression.
Depending on your experience all this may or may not be noticeable and they are probably "ok" however if you are not looking to splash the cash (and better than what Edmund Hillary had back in the day) and will last your trip and probably a while longer. about 1 year agoAnswered by Sean via WorldNomads.com
I went in Dec-Jan with just a cheap (real down) one and only ended up using it in the afternoons/evenings after finishing the trek for the day (which I found much colder than early mornings).
I would make sure you can fit at least 3 layers under it and a hood is quite nice to keep your neck warm, but I wouldn't worry about getting a fancy one. 9 months agoAnswered by Soph via WorldNomads.com
A light down jacket is more than sufficient. I would get something that is lightweight and around 800 fill. I brought an Arc'teryx Cerium LT Down Jacket, which 850 fill and weighs 8 ounces. Nepal is all about layering when the sun is shining in the mountains you'll be happy with just a tshirt. Make sure to bring a rain shell as well. 6 months ago