If you have not already bought FoV, you may be able to get DKS cheaper (Steam and Amazon, etc, sometimes put it on sale). If you get FoV, there is a large
conversion patch to turn D2:ED+FoV into DKS.
With FoV you can install the add-on, start the game, load your last save in the Hall of Echoes, finish off that fight and continue into FoV with the same character.
DKS includes the remastered D2:ED (redesigned flying fortresses, changes to enemy placement, changes to the loot and enchantment systems, etc) along with FoV, but unfortunately saves from the original D2:ED are not compatible. You could start a new level 35 character at the beginning of FoV, though (as you can with the original D2:ED and FoV installed).