Bernard here's some good advice.
Collect at least 300 souls everytime and use it in the shop by 100 souls purchase like Kartik said. It has better chances to get 1 piece at a time if you're lucky.
If not lucky then forget it, you don't need the shop 😄. All you have to do is play in inferno mode non-stop for weeks to a month and you will complete the inferno set. You already have the key part (inferno sword). Other parts of the set drops in that area particularly boss stages. This requires patience, perseverance and lots of play time.
I hope this helps and have a nice day.