Yeah, I can't crawl under a W123 on the ground either, and I'm not that large. I can slide under it fine, but then have no space to work.
As far as bleeding the system goes: after you put the heater hoses back on, remove the top radiator hose, and fill it up all the way with coolant. Squeeze it a couple of times while you're filling it up as well. If it starts to bubble and the level drops while you squeeze it, that means you're burping the air out. When the hose is topped up, put it back on the radiator. Then make sure your radiator/expansion tank (not sure which one you have) is at the correct full level and run the engine without the radiator cap on, until it reaches operating temperature. Blip the throttle a couple of times while you're waiting for the engine to heat up. After it reaches 80c or so, check the coolant level through the filler hole and if it isn't bubbling, put the cap back on and you should be good to go. It would be easiest if you can get the front end lifted up a bit. Either drive it on top of some wood, or ramps perhaps? I personally wouldn't start an engine with the car on stands. Those diesels shake a bit too much for my comfort.

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)