[rhandwor]

If buying a used head I would have it pressure tested and checked for straightness at a machine shop. If slightly warped they can shave it slightly for you. You can do it without doing this but always run a risk of it leaking.
A lot of machine shops will sell a rebuilt head. They will go through yours when business is slow.
A lot depends on what you buy if buying a used head from a running engine without leaks you probably can get away with it.