Did you bleed the system after replacing the fuel filters? Does it have the original white bleeder pump or has it been replaced with the black plunger? If it is the original white one, it could be sucking air into the system. If you have not already adjusted the valves, you will need to do that . If they are tight, that will keep it from starting. It sounds like your GP relay needs to be replaced for sure. If it it cold, and you did get the glow plugs working manually, you will need to crank it until it either starts, or the battery runs dead. The starters on these are built and designed for long cranking times and it will not hurt it. If all else fails, I have had diesels that have sat for many many years that I ended up having to pull start the first time to wake it up then it started on its own after that.