what would be the best donor vehicle?

I think it depends a lot on personal preferences and how you plan to set it up. Some people don't like the look of the truck intake and cover so they want a car engine. Others prefer the accessory layout of the truck engine. Some don't like the extra plug wire on the 03-05 5.7 engines and prefer the 06+. Some people avoid engines with MDS, others want to try to keep it. Some don't want an 09+ because of the VVT. If you aren't as picky you are most likely to find the best deal on whatever was most commonly produced in your area.

If you are planning to use the factory computer and an old manual transmission you might want to look for a truck setup or at least the computer from a truck with a manual trans. More truck were built so they are more available.

I ended up buying a 5.7 truck engine and transmission out of a wrecked 05 Ram 2500 with 8000 miles off e-bay for $2000. Some plusses for me personally are that it does not have EGR, the 05 computer allows for good tuning options, it doesn't have MDS if I want to up the output in the future, I should be able to run a manual fan, and with some floorpan surgery I was able to get a modern overdrive trans. I'm happy with what I have, but if the right deal would have came along I might have bought something totally different.

In my opinion, a new hemi swap is generally not easy if you want to keep it cheap and is not very cheap if you want it to be easy.
I'm trying to do some things on a very tight budget so I have to do more work in some areas where others might just spend the money to save time and make it easier that way.
How you choose to wire it and whether or not you go with a carb or EFI make a big difference in this area.