I didn’t include a rule to prevent buying ahead with indulgences, so you can do it if you like. As you say, I did try to be explicit about stating when another purchase is a prerequisite but I also understand that there is a bit of ambiguity here. Ultimately though, it’s your game and you can play however feels right for you :)
edit: oh and as for the level up purchases, it was my intent to buy them in order but I can see now that I was not as clear in that case. That said, with the cost increases being what they are it’s pretty fair to play it either way I suppose.