Trip to Tokyo, fees and technical questions

As I'm still a student, I'd like to know how much money should I have to go visit Tokyo, especially Akihabara as I'm interested in retro gaming and I collect JP games.

-how much would cost me the cheapest accommodation?
-how much do I need for food?
-flight tickets prices are easy to check and as the EU citizen I don't need a visa so that's covered
-are there some other fees I need to worry about?

And some technical questions:
-should I get €, pounds or $ and exchange it in JP or do I need to come with Yens already (of course I'd try to get some before going either way but I think it might be hard)
-can I use European bank cards in JP? as of now I have Polish and British bank accounts
-can I send a postal package with souvenirs back home?
-is it easy to get a cheap pre-paid SIM card for my cell phone for few days I'd stay there?
-do European cell phones work in Japan at all?

Thank you for response in advance.

I came with Euro .After Landing in narita I exchange it into yen.So don't worry.For Food= I went Buffet restaurant called sweets Paradise And eat unlimited Food and drinks for 1530 yen.so monthly cost on food is 45900 yen for myself at minimum level.I bought 72 hour Tokyo subway pass=1500 yen.so monthly cost is 15000 yen.Hope this helps.

Those are pretty much the most common questions asked here and elsewhere (well, after the "itinerary review" ones). Do a search.

If you'll be shopping in Akihabara, the sky's the limit as far as how much you might spend.

If you want cheap accommodations, the cheapest is probably a hostel or a guest house, price start from about 1900yen at Asakusa.

Food can start from about 400-500yen for a curry rice or ramen which is on the cheap side, convenience store also have cheap food.

As for cash, exchange what you think is enough for you, and have a credit card or a ATM cards for backup, the usual ATM cards are Visa, Master, Plus and Union Pay, and remember to activate all the card for oversea use in you home country, activate them at least 4-5 days before use and in some countries you can have this done at a ATM machine or through a bank, and remember activation cannot be done out of your home country.