General advice for Scandinavian part

Hello everyone,
I am preparing for E1 journey next summer (2 months) from N to S but heading to Stockholm via Falun. I need an advice regarding the size of the backpack which is connected with lower resupply availability. I am considering buying Deuter Air Contact 75+10. Will it suffice? Is there a list of places where you can buy food and how much cash should I bring for the “wilderness” part (for food only)? I´ve found in the previous post en excel file but there are some question marks so I wonder if anyone have some additional info?
I am also wondering about path marking. Is it necessary to have a GPS still turned on or is the path obvious? I thought that I take only my cell phone with the app mapy.cz where are all the main paths. But I am not sure about the battery if it needed to be on for a long time. Also there were mentioned some river or creek crossings, how deep or large are those? Do you need to take of your clothes and get across or is it shallow? Regarding camping do you suggest to stay on described places (on this web page) or is it possible, if I manage to go further, to build a tent on differents spots? I imagine there are some places (rock fields, swamps) where it is not possible, but generaly?
I will go in july and august. Thank you for any info or tips .
John
Czech Republic