As a test, take a 8 gauge wire and connect it to one end of the glowplugs, and the other end to the + terminal on the battery. Go try and start the car. From the time you hook it up, give yourself 30 seconds before turning the key. If it starts better, bad wiring, fuses or relays. If it's the same, glowplugs, timing, injectors or compression. Or a combination of things. You can leave the car running and unhook the cable. I think the glowplugs are on for up to 2 minutes on the AAZ so you aren't going to damage them.
It's very unlikely that the rings are bad after 75k km.