340 X heads advice

Dependent on machine shop you are using will depend on cost. If heads are used plan on valves, guides and new seats no matter what so when the machine shops quotes you.... you are not surprised.

As for what to look for - bring a good straight edge and feeler gauges and check for warpage on head gasket surface. If you don’t know what I am talking about then google that plenty of videos online. Also look for cracks between the valve seats or seat edge going outwards.
Check spark plug threads. Look for the area around the bolt head mounting faces for dug up or gouging- over tight. Over all look at the heads- lots of coats of paint mean possibly they have been on multiple engines. “X” heads are not as hot now as they used to be because of the amount of good new heads available now. A good budget for heads for stock should be about &1500-$1700. Including cost of heads. So I’m not sure what you are spending keep that as part of the equation- remember cylinder head work can be as expensive as you want it to be. Good luck!