My $3 Harbor Freight multi-meter does just fine. I typically test the plugs right at the plug itself to rule out any harness problems.
I don't see the point of grounding at the battery. You're testing the resistance of the plug so I would think it's better to ground to the engine block since that is what the plug is grounded to. Why introduce the variable of the engine ground strap into the equation?

I use fantastic gear wrenches to remove my dream glow plugs and actual gear wrenches to remove actual glow plugs. If things get really tough I reach in my toolbox for the Platonic Form of Gear Wrenchness.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13