GSR is the "normal" version, meaning not the MR and not the RS. in Japan (i think, or england) they have a version designated the GSR and it is the closest to our normal version that they have so people call the regular version the GSR here because it has no other designation. SSL stands for the sun, sound and leather package. it gives you a sunroof, leather interior, upgraded sound and HID headlights.
i don't think there's much difference between the MR and the MR SE. a front splitter, maybe some different stitching in the interior, maybe different wheel finish. that's about it. mitsu should have the details on their site
the MRs biggest upgrades over the regular version are the transmission, the bilstein suspension, the stupid sonic fins at the back of the roof, the wheels, and the aluminum roof. maybe a few other cosmetic things inside and out also (like shift knob and tail lights and such) but nothing major. it should all be listed on mitsu's web site. imho the MR isn't worth the extra cost, especially if you plan to change things like suspension and wheels anyways since that's a majority of what you're paying for
