They are not all the same. The <10 dollar Harbor Freight one (sometimes as low as two bucks on sale) will do basic DC and AC volts and resistance, but not with great accuracy. I have one and it's good enough for working on W123's. From there they range up into Flukes with four figure prices that do all kinds of stuff including AC frequency measurement. A Navy Reserve colleague of mine who is an agricultural equipment mechanic in his regular job has a $1700 Fluke that you can take over small countries with. I think it also doubles as an oscilloscope. I'm not sure what parameters you'll have to measure to troubleshoot your 300E. If you know from reading the FSM what you'll need to measure, check with various tool dealers to find what you'll need at a price you can live with.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.