It's all good advice, but to recap, here's my contribution.

Get the manual. DO NOT touch anything untill you get the manual. Read the relevant section as many times as it takes to understand what's involved in the next step that you need to take.

Try to buy the whole gasket & seal kit as one. This saves money & a lot of dickin' about buying separate parts.

For the extra dollar or so, buy brass core /freeze /welch [pick one] plugs. Always use brass ones in locations like the back of the head & behind the flywheel.

Datsuns will wear in the bore, but the crankshaft is almost bulletproof. If the engine has oil in the pan, you won't kill it, but if the journals do prove to be worn, then these surfaces will need to be ground down to the first undersize. The bearings that go with the re-sized crank journals are thicker, & are often refered to as oversize. You put the correct oversize bearings to the undersize crank.

Just follow the book & you will quickly determine wether you will be able to go the more economic route with a major overhaul [quite likely] or if it is too worn, then a total recondition may be needed, but if it runs OK, but is just a little tired, then the overhaul could be the go.

What's the diference?
Major Overhaul = Return all clearances & mechanical specs to within "service" limits.
Recondition = Return all clearances to within "new" engine specification limits.
Rebuild. This word is widely used to describe engine work,.. but what does it mean? It can mean just about anything you want as i have never seen a proper definition of its meaning.
"Rebuilt engine" is a phrase often used by charletans, crooks & other snake oil salesmen.