Tickets generally shouldn't be a problem although Krefeld is a derby and March must be more or less the last game of the regular season so might be harder than some games. They play out a huge arena that I've seen sold out in the derby v Cologne but barely a third full in a league game against Iserlohn.
Atmosphere is ok but certainly not as good as other clubs (Cologne, Ingolstadt) that I've been too.
Getting there is not too bad. Train out to Rath station I think it is and then there are shuttle buses from there to the arena. I think there is a bus form the centre of Dusseldorf straight there but it takes some time as it is in the sticks.