In case OP is still around I want to offer my input. Also for anyone else considering a Ford F350/450 platform
So, I've been trying to find the right platform for my skoolie. Given that it's just me and my harps (gotta have a spare) I'm thinking about converting an E-450 Cutaway.
Cool, I have a 2003 E450 shuttle with the gas V10. This platform was used for shuttles, Class C motorhomes, box trucks, and a few other dogs and cats. Very common.
That is entirely up to you and your results
How many miles is too many for the engine? Same for transmission?
Honestly, around 20 years old or 300,000 miles things tend to fall apart all over the vehicle and it comes down to your desire to keep it together. I have a 2000 F250 with V10 at 261xxx miles and she is running but showing her age. Ford puts a lot of plastic and whatever cheap(er) parts in this generation truck than previous models. The upside is the modular gas engines get better mileage than the previous generation big block engines but both can go ~500K miles if maintained. They will be tired but should be able to reach that odometer reading. Most owners give up long before that mileage milestone however.
If it is getting elderly, what can be done? Valves? Rings? Throw it out?
Change the oil and fluids on time, keep the tires and brakes healthy, and drive it. Sitting can be worse than high mileage, I am currently evicting mice debris from my bus after it sat for the last year.
There were crate engines from Ford available some time ago but I don't know if they are still available now. Several shops have tried their hand at offering rebuilt modular engines over the years with mixed success. The biggest problems came from a lack of maintenance (oil changes) as the engine needs clean oil to maintain healthy timming chain guides (all generations) and in later years (mid-2000s 3 valve and up) adjusting cam phasers were introduced which needed clean oil changed or they could fail.
Transmission shops are very familiar with the 4R100 transmission if equiped, I don't know if the Allison was fitted to any E350/450 series.
Pardon my ignorance, but I'm a retired commercial pilot, and we never cared about the date on the name plate, we wanted to know about the components.
I am at 30 years as an aircraft maintainer. In the business as you know we have rebuilt components available as long as a platform is utilized. By comparison the auto industry moves on very quickly and the OEM will offer parts for about a decade after a vehicle is new, after that time the parts will be discontinued as OEM and you are left with whatever the aftermarket chooses to offer.
You also asked if diesel engines were available in the E450. The answer is yes, the International T444 series known as the Powerstroke in Fords.