You can check surface flatness yourself. Most A12 do not need head skimming (head milling). Use a straightedge and a .004 inch feeler gauge. It is allowed to be up to .004 from flat but I've never even seen that much even after overheating and blown gaskets. Normal is .002 per the Datsun factory service manual.

For head gasket to last, preparation (block and head face cleaning and bolt thread cleaning) is the key. Also , no oil should be on the bolts or on the faces.

Valve seals are easy to replace, if you have a valve compressor, it is trivial.

You can wiggle the valves to see if the guides are OK (this is the part most often wearing out on A12). There should be almost no discernable wiggle with the valve slightly off the seat. If there is, replace the guides.