I have an 02 mc that won't start, my speedometer doesn't work when it was running and would over heat just going up the road. The battery has been arched from previous owner but I've changed that done a couple of jumps for people and changed the alternator. I don't want to get rid of the car. I want to find out what's wrong

 

Not starting is most likely one of three things, if you're sure the battery is ok:

Fuel pump
Fuel pressure regulator
Starter motor

Overheating is most likely the thermostat, unless you have puddles, in which case it's the water pump or a hose.

 

When I check the coolant its a bubbly grey water type.

 

3400 or 3800?

It's not super common, but you might have a head gasket issue