Mobile internet for 3 months?

I will be staying in Japan for 3 months and would like to have some kind of an internet connection while in town and to be able to send messages/Skype call to my friends at home. What do you think would be the best solution for this?

I've looked at the pocket WiFi services, which are quite expensive (400-700 euros) considering my longer stay. I would be willing to pay 150-200 euros max. I also read about actually buying a mobile WiFi device from the internet and then getting a SIM card for it in Japan. How does that work and should I even consider that in my situation? And is it possible to get a prepaid SIM for 3 months, the ones I've come across were all for shorter periods of time?

I would rent a wifi unit. You can get them for much cheaper than what you've found. I'm currently renting a wimax2 + LTE (unlimited data, though they throttle you way down after 10GB/3 days on the wimax, and after 7GB/month on the LTE) for 60 days for 14000 yen. 90 days would be 19000 yen, or about 160 euro. I got mine from Kashimoba (https://www.kashimob.com) which was the cheapest I could find for something with good speed and generous data limits. They also have cheaper plans for just the wimax or LTE. Might be tough to navigate if you don't know Japanese though, as it's aimed at the domestic market and not foreigners. In my search I did come across a service marketed at foreigners with a similar deal but was a bit more expensive (forget what the name was now.)

Prepaid SIMs can usually be extended at the end of their usage periods, or you can just buy a new one, which amounts to the same.