The SAN @badders has will be 3-4 years old if it is still the one that was got when I worked with him :) So adding disks is one option, but as you have said considering age, warranty etc need to be considered. @Domino has quite a lot of these things dotted about and if they are anything like HP Servers if looked after IE kept in the right conditions they will probably just go on and on and if you can swap the controllers for newer units that means your nearly changing the main parts of the system so could be worth looking into those options :)
From my viewpoint with that I would add a disk tray or look at EMC kit, I know @kmount and @linescanner at Cutter have used some EMC kit and had great performance and I think if I was SAN buying now I would be getting an EMC unit.
New EMC VNXe 3150 or the 3300 are good bits of kit. EMC are quite aggressive with the pricing in to Edu also.
Not sure I would bother with Fibre