Darren, the van's history of evident hard use and poor maintenance certainly shifts focus on these issues. All my SO experience tells me that you might have already SAVED this van from a premature death.
When it suddenly overheated in the drive-thru, did it leak coolant, or did the engine temp just suddenly spike? If it has a mechanical belt-driven water pump, they usually fail gradually. Typically you begin to notice a coolant leak, you can smell it and see wet spots on the engine and on the pavement where it's parked. Often the pump makes noises, this is all a warning it's on the way out.
Being a Transit van, it probably was in hard commercial service "rode hard put away wet." Too often commercial vehicle owners don't give proper care or maintenance. MY late brother and his second wife had a great little 1980 Dodge 1/2 ton step-side pickup w/ a Slant Six and 4 spd-overdive stick shift trans. I borrowed and used it several times, and often did needed maintenance on it. But with a small print shop and 2 sons to raise, the faithful Dodge sadly was driven into the ground. I once did a major brake job on it- ever seen a brake drum come off in pieces...?
Not too personify your Transit, but you and Snake Oil are the best thing to happen in this handy van's life! The war wounds it has suffered need to be addressed so it can give you long service. We use vehicles instead of horses and mules today- but like critters, they need good care to last.
I agree w/ Ron and Greg K on the cooling system. Today electrical components are replacing many formerly mechanically-driven parts. Complexity is ever-increasing. Service intervals on most everything have increased- and this can lead to complacency on owners' part.
Keep us informed on the Transit, we have things to learn together w/ this one!
Tracy G