You're gonna want to do some research on best engines and transmissions and then find something that is good based on that. An old post of mine is stickied somewhere in the forum from back when i did all my research before buying. It might have been under my older username that I forgot the password to.
I have a DT-466 engine with the AT545 transmission. I didn't know it had the 545 or I wouldn't have bought it, but it was sold as an Allison 2500, and the mechanic I had check it out for me didn't tell me otherwise. The 545 makes climbing hills a bit difficult. The engine is amazing though. Mine is a flat nose front engine, and I would never do that again in a million years. Super loud, hard to work on. Get a dog nose.