The el-cheapo Harbor Freight kit comes with both the injector adapter and the glow plug adapter for the pencil-style glow plugs. Shell out the $20 or so it costs and find out what the compression situation is. Is it a cheap and nasty kit? Sure. Are you a mechanic in a garage that's gonna use it every day for a living or are you just trying to diagnose a potential problem once? If the latter, don't shell out huge bucks for a basic tool you're gonna use once. You're more interested in the balance between cylinders, the precision of the peak pressure recorded isn't nearly as important.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]