Bulletproofing can mean a lot of different things on the 6.0. BPD apparently think they have patent rights on the term but they do have the best EGR cooler.
What does it mean to Bullet Proof an engine?
Also something to look for and/or indicate to your prospective buyers is whether or not you have a coolant filter and if you switched to an ELC coolant. If it's got an EGR delete you would want to mention that too, especially if the buyer is located in a state or city with emissions testing.
Either way, if the heads are leaking then you most likely need head gaskets. The engine has to come out. Count on 30 to 40 hours of labor on a van and with parts (new headgaskets, arp studs, oil cooler, updated turbo feed and drain tubes, STC fitting, turbo rebuild, oil, coolant, new belt, blah blah and all the other things you'd probably want to do while the engine is out (new engine harness perhaps) then yeah, 10k is not out of the question.