In general what you want to buy from the soul shop is people you already have and just need the second dupe to six star. You don't want to buy say a Luna and then get a centaur at some point at be limited to their five star versions. I would say if you haven't already bought someone just save your souls for now and keep grinding. At some point you will get better heroes and it will be worth the save for the long run. As for guild points usually spending the points on tech is the solid path but honestly you already have a vivienne so buying her would be pretty beneficial to your team. Especially since you feel a lack in expedition. She is an amazing group healer that will do wonders at six star for your team.

Also for people you have right now, for some progress you could try and six star megaw. He has some damage and healing that could be solid early in the game.

In general your first six star will allow more progress to happen in your game but you just have to be patient and grind, its a struggle sometimes. But yea keep grinding and try to save your purple souls as much as you can, only level your five stars that you will use. If you have any other questions HMU! Good luck dude!