If after initial start up the motor runs ok,I would suspect air being drawn in to the fuel line delaying the start,copper washers on filter banjo unions are prime suspect.
Best way to test glow plugs is to remove them,resistance across a removed plug is 0.4 ohm.You can also connect up briefly to the battery and see the glow.
With glow plugs in,connect a voltmeter to earth and the glow plug terminal.
Set the meter so you can see it,switch to posn 2,you should see voltage for a minimum of 3.5 seconds while the glow plug light is on and a further 3.5 secs after it goes out.If this doesn't happen,the glow plug relay or fuse can be faulty.
Just another thought,the valve clearances are adjustable with shims,as it's a 94 and perhaps a 100,000 miler,you may have a tight tappet or two,losing compression.