For two nights storage you might be best to leave the van at the Cvan park in Bega.
Staying in Jindabyne is normally cheaper than Thredbo, but you miss out on the stunning views and atmosphere. The older units on the side of the hill in Thredbo have much better views, but the new ones down in the valley are obviously going to be in better condition. My old favourites were the ones just under the Alpine Way(higher up the hill) harder walk up and down, but lovely to sit and look out across at the ski runs.
You can walk from Charlotte's Pass to Kosi, or take the chairlift at Thredbo and walk from there. Charlotte's is longer, but it is an old road so slightly easier., whereas from Thredbo you have the metal grate path. Pluses and minuses each way, really. For a first visit you will want to take the chairlift anyway for the experience, so you might as well do the walk from there.
Take wind proof/rainproof jackets and sun protection and snacks for the walk. They will tell you the weather changes rapidly up there, and they really mean it.