Yes, you can find medium duty fords with Cummins power. The contract only gave Dodge exclusivity for the light duty sector.
The dodge transmissions are not bad. They are not great, but you can get a good lifespan out of them and rebuilds are cheap.
The allison isn't un killable or anything. In fact, the guys that really go all out performance on a duramax swap in a built dodge trans.
At stock power the dodge is decent. Many people can get at least 100k out of it, sometimes 200k. If you want to run around with 500 hp 1000 lb torque you will need a strong trans build to the tune of about 5000+ but they can be built to hold up. The cool thing about a cummins/duramax is you can really decide how much HP you want and set it up that way. The bad rap on dodge trannies usually comes from people running a lot more than stock power with a stock trans and yeah that breaks them quick.
The only bad thing i can say about a Duramax, is people don't usually sell them for any kind of a good price. The duramax is only about 50% as reliable as a Cummins but the rest of the truck and trans make up for it. But it still a solid truck if you get around 2006.