1) For an I.T. job Business japanese is almost an essential they won't take anyone who can't understand the O.S, Drivers, Proggies etc. And for programming you need to almost be Fluent.
2)If you want rights then yes you do need a workers permit or you will either be working for Yakuza at the docks/Clubs/(some) Restaurants/Bars OR you will be working for Student wages (around 750 yen per hour) Or a dodgy Englsh school/daycare center which pays shit with no proper equipment.
3)Japanese are very anal about everything so yeh, the ones that aren't are usually not very secure jobs because they get found out quite quickly unless theres illegal pay offs going on somewhere.
4) I'd say around 3,000 euros should suffice to get you started that should last about a month. Most places your going to be staying the rent is going to be around 200 euros to 400-500 euros depending on whether you want shared accom or your own unit thats for a month most of the time though.
5)
Jobs in Japan: Jobs - Japan Jobs - Japan News - Japan Info - Japan Apartments - Japan Classifieds - Japan Forums is a good one that or check in with your local japanese embassy.