When you take your old heads off you will know why people want to put new
heads on.  The old heads will, at the very least, be very corroded around the
edges.  You will have to have them resurfaced in some way (either welding or
J B Weld).  Also many used heads have cracks between the valves.  I always
have used heads pressure tested by a local shop to be sure that even small
cracks don't go up into the water jacket (can you say, "combustion gasses in
the coolant"?).  So if you have the heads welded and pressure tested, you are
looking at around $150 worth of work on old heads that still have the same
valves in them, etc.  And your down time increases as you wait for the shop
to get this stuff done.  

With new heads you get all new valves and everything.  The head surface is
already clean and ready to put back in.  The only new heads available now are
made by AMC a company in Spain.  The heads are good quality though and I have
never had any problems with them.  The alloy of the heads is actually
supposed to be better than original too, to prevent more pitting in the
future.  I sell these for around $395 a head.  I also sell the complete
engine gasket kit for $120 (includes that pesky tube of yellow head nut
sealant that usually is left out of these kits).  I hope this helps you.

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242