It's completely possible to piece together without a donor car. And if you know your way around the cars, it can be an excellent way to do it. I have several times.
BUT, I will still say that generally-speaking the best way (cheapest, lowest risk of failure, most complete) is a donor car.
I'm in the middle of a Lexus 1UZ 4.0l DOHC V8 swap for my 4Runner. I bought the engine out of a car, and could not hear it run. So I bench ran it before I bought almost any other parts for it. Those ECUs are notorious for having failing capacitors, so I replaced those, and bench ran it again to make sure it still worked. Then I coated an exhaust manifold and ran it again. All together, it's probably run about an hour on the stand.
Skip the first 10 minutes to hear it run.