If I was you
@jimmybell I'd source an engine with as much of a back-story as possible, which would probably mean prioritising a 'forum' engine over an unknown engine from FB or eBay, even if it has higher miles. I'd rather have a 120k engine that's had regular oil changes with good oil, and taken to the redline every now and again, than a 60k engine that's been driven to the shops and serviced every 12k.
There's an interesting video on the Bathtub failure distribution model when it comes to replacing parts, it's a Land Rover video but the principle is the same:
I'd get that used engine delivered to Alex, with a brief (assuming your funds allow) to health check and build in reliability while it is on the engine stand out of the car. Subject to a good compression test I'd avoid opening the engine up if I didn't have to (for reasons explained in the video).
I'd have the block cleaned and then instruct the replacements of gaskets, seals, oil and water pumps, spark plugs, dephaser pulley and all the belts, and new fluids obviously. Depending on the state of the idler pulley (these seem to have a 'life'), I'd get that done at the same time. You've got an opportunity while the engine is out to do all the awkward stuff, so it would be a false economy to stick an unknown engine straight back in without some preventative maintenance and TLC whilst it's all in bits, whilst keeping any known good engine internals intact.