Cold weather starting issues, signs of starter wear?

[tomas_maly]

I'm noticing with my 1983 300SD that it's been harder to start in the morning when it's 25 degrees F out. I haven't changed or tested the glow plugs since I purchased the vehicle (6 months ago), but have noticed that turning the glow plugs on several times doesn't help and that starting the vehicle requires holding down the ignition key into 'START' for probably 30 seconds. The engine cranks and slowly, slowly starts gaining a higher pitch until it finally catches and ignites, when I need to then keep the gas pedal down for probably another 30 seconds before it will stay on without stalling.

Besides changing the glow plugs, is there is a way for me to recognize (or test) that the starter is wearing out? Maybe something using a multimeter? I have two starters, one on a running vehicle, one on a dead vehicle, and I was hoping to check both for quality.

If the starter is wearing out, is that something I can repair, if I so have the ambition (and electrical knowledge)? I have a spare starter, but there's a chance it is somewhat worn down also, and I was hoping to get it into an optimal condition onto my vehicle this winter.

Is it normal for me to have to hold onto the gas pedal for 30 seconds before the engine will stay idle/on without stalling at a cold temperature? Or is that a sign that the glow plugs aren't doing their job so well?

Should I be turning the glow plugs on 2-3 times, and will that make it start easier (without stalling)?