I think this is a better way to go.
To me, buying an assembled engine from an unknown is sketchy. You don't know what parts were put into it, and what was done for machining. Sure you can ask, but it doesn't mean you're getting told the truth. Not that they're all bad guys out there, but there are bad guys out there.
With a builder, you can see what's being done and what's being installed as it goes. It will probably take longer, but it can be built for your requirements, not something generic.