Head Gasket

Done the head gasket on mine a little while ago. 
It's pretty much an engine out job. The sump needs to come off to get the timing cover off, and reassembly of the timing components is going to be difficult with the engine in. 
While the engine is out it's not much more effort to get the head (and in my case the block as well) machined flat, as it will have warped if it's been overheated and the head gasket is gone. In my case the head gasket had gone between the cylinder liner and the water jacket, so the block had to be planed for it to seal again.

It's not a small job on these unfortunately 

The thick orange coolant will be due to all the K-seal that the previous owner put in to hide fix the head gasket leak, so it has lasted whatever mileage you've done since you bought it.

I don't see why you can't take the timing cover off with the sump on, and anyway you can remove the sump by lifting the engine slightly, but I agree with Hughes, the amount of awkward dismantling you have to do it will be easier to pull the whole engine out and get it on a stand.

Loosen the crankshaft pulley bolt before you take the engine out.....

I've done a few and see absolutely no need to remove the engine. The sump stays on, it's just the front few bolts you need to undo. Timing was no issue, visibility is good enough with the radiator out. 
I got the heads skimmed as a precaution. It's not too expensive. It took me about 10hrs all in, split over two days to allow for the head to be skimmed.

And my experience aligns with Lambert's. Once an M series engine has overheated the head gasket will need doing sooner or later.

Pay extra attention to the cylinder liner deck surface (for want of a better word) between cylinders, if it has corroded / pitted between the cylinder and water jacket it will not seal properly again and you'll have a compromised head gasket. 

That's why I took the engine out of mine and had the block planed.  Also did an overhaul seeing as I was dismantling it: Rings, bearings, timing chain etc.