Hot upper hose and cold lower hose could be a clogged radiator, stuck thermostat or cooling system pressure overcoming the pump. The #14 head doesn't like it when the temp gauge goes beyond 120*C so that alone is reason to take action. You might try a new good quality thermostat before pulling the head. It'll still be new when you replace the head and/or gasket A new thermostat might balance temperature across the radiator but it won't fix system pressure.

If you need your car in the meantime, run it with the pressure cap loose like on the first notch. Cooling capacity is greatly diminished without the ability to pressurize but you won't boil off coolant in normal driving. If you'll address the situation shortly, run a reduced coolant mixture like 10-20% just to keep it from freezing. Top off in that proportion but you probably won't lose coolant if you keep the pressure cap loose.

Have the #22 head checked by a knowledegable machine shop. At the very least get MB brand valve stem seals. If the PO ran WVO or drove like a granny, have the injectors cleaned and pop tested. The prechambers too if you can get them out. Maybe the machine shop can pull them for you. Plan on using the injector lines from the #22 head for best alignment. There's a spec for head bolt elongation. If they're within spec, you should be okay reusing them. You'll need a couple of longer bolts that hold the head to the front cover. They're nothing special. Look how much the short bolts extend from the #14 head then look for similar hex head bolts at the hardware store that will extend the same amount from the #22 head.

The head bolts take a triple square/double hex bit. I think in 10mm size but maybe they're 8mm. I use a cheapo 4-piece bit set from the auto parts store but there's a fancy bit available as a MB special tool. The special tool allows you to pull the head with the cam in place. A klutz like me should never handle the head with the cam in place which means some valves open so there's no question to me that the cam comes off before anything else. Then I can use the cheap tool. The special tool to remove the valves probably runs into 4 figures after the 2014 MB price increases. A watercooled VW valve spring compressor might work. Otherwise, leave it to the machine shop. Don't pull the cam followers with a magnet as the FSM suggests. A later TSB says to use a small suction cup such as a hand turned valve lapping tool. A magnetized cam follower can trap ferrous bits circulating in the oil. You'll need a special splined socket and slide hammer adapter to pull the prechambers. A timing chain guide rail pin puller is invaluable IMO but you can get away with a bolt that'll thread into pins and a stack of washers. Injector heat shields work well as stackable washers.

For parts, use a head gasket set for your 3.0 engine, not a 3.5. The #22 head uses slightly shorter glow plugs. IIRC Bosch 80031 vs 80006 for the #14 head. Don't quote me on that, just remember they're different but look interchangeable. This is a good time to ensure you have a full set of injector line clips and hangers.

This is out of the W124 FSM but everything applies to your engine - http://www.startekinfo.com/StarTek/outside/12264/Program/Engine/602_603/01-5800hx.pdf . You can access the FSM online at www.startekinfo.com at no cost. Registration with a valid US credit card is required for the EPC but not for the FSM.

Sixto
87 300D