How many skills/items/upgrades can I buy in a single playthrough

Ok, so after several tests, it appears that yes, this achievement has to be earned in a single playthrough.

Also, I had a little problem with Granny Rags: during my second playthrough to get all the paintings, I ran as fast as possible, skipping almost all the side quests, including Granny Rags ones. It prevented me from getting the last painting ("The Outsider and the Circumstanced Void"). As a matter a fact, when you skip her quests, you cannot access one of the last areas where you can either fight her or help her to kill Slackjaw but also access her room where you'll find the painting. So that's something you'll have to keep in mind if you start a playthrough dedicated to the paintings.

You can't see it in the menu or anywhere else. The only things you see is:

• If you find one collectable, it will display your total amount of collectables from this type in the bottom line.
• If your done with the level, it will provide a summary of all found collectables and those which could be found in the level.

But you can take a look at a list of collectables per level to get the info early enough and just remember how much you already found in a specific level. Such a list can be found here.