The bus is a good idea if you can deal with your luggage on it. How much do you have?
The Kings Cross St Pancras tube station is multi level and well served by lifts as well as escalators. That is both good and bad news. Because of my wife I have to use those lifts. If you plan to use them leave a little extra time. They can be confusing and they only go up one level at a time so to go all the way out or into St Pancras International will take some walking and finding each lift you need.
There is a bit of a ramp from the St Pancras tube booking hall into the station. If your luggage is a but difficult I might suggest going up the Kings Cross side and walking across the little street between the two stations. The Eurostar are is about midway in the long and narrow St Pancras station, on the Kings Cross side.