Looks like the Soulfused construct advances just as the Soulspark familiar does, which gives it a boatload of essentia. At a rate of 4HD/CR bump, and +1 Essentia/HD At +3CR, with all ability boosts going into Con, it can start shaping Soulmelds.
Alternatively, if you did decide to use class levels, it would have equipment, as well as start with the elite array. Another method would be to give it an inherited template to increase Con. Half-dragon nets us a Con and Int boost, as well as some other stuff. (Such as replacing the darkvision the poor little construct lost).
Also: The Guerrilla Warrior feat from Heroes of Battle makes Hide and Move Silently purchase as class skills, I know theres a way to get them to your class skill list, though.