Roughly how much would if cost me to get a head reco'd if i bought a used one? Things like checked and tested, resurfaced? Thanks

Checked for hardness + pressure tested is normally $20-30, skim around $50, reseat $80-150 higher for better valve jobs, $20 for stem seals or use them out your h/gasket set, then if you need valves, guides, seats or welding prices can climb quite quickly.

As mentioned, it depends on the extent of the wear/amt of parts/work required. If you are going to do it, be a little weary of the cheapest places, you usually get what you pay for (though in some instances you can pay a lot and get ripped). If it requires the full monty - new guide inserts, new valve seats and valves, given the cost (you'd be looking at a good few hundred) it'd make sense to enquire about getting some porting work done at the same time. It'd raise the price, but if everything has to be replaced, you might as well improve it. It doesn't have to be full race porting, just work on the valve/seat angles and the bowl area will net big improvements.

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.