I would have to say, it really comes down to how much you really love your expo or if you could live without it. Mine is an 02 and I love mine, but on the other hand the blue book value vs the cost of repair is where it gets me. Since I love mine as much as I do and it shows (Mine is in excellent condition), I would probably put money into it if the cost were under $1500. If it were the head then I would swap the entire motor.
The biggest repair I have had to do on mine thus far was the intake manifold. She hasn't been the same since! However, when the manifold blew, I couldn't afford the repair. So she sat for a couple of months and then I had an old friend that everyone calls, NASCAR. He earned this name from building rat rods! So I was confident that he could don the job right. Well, as I said, "old friend", he doesn't think the way he did when he was building rat rods. Unfortunately, the truck had no power and she had to be torn down a second time and by a different mechanic who claimed to have a degree in auto mechanics. Well he had a degree, but turns out he never went anywhere with that degree after he graduated college! He went to work with his father remodeling houses! So he didn't k ow what he was doing and his mistake was not replacing the crushed gasket with a new one and it had some serious air leaks! Never even test drove it after he was done. Started it and shut her down and started over with round 3! This time it was my boyfriend that did the work. We bought another brand new manifold and gaskets as well as plugs, wires, coils, injector seals and for good measure changed all the fluids. This time it was good!! It took 9 months to get her back on the road!
I have always been picky on my choice of wrencher that gets under the hood and this is exactly the reason why! I have the know how to fix stuff myself, but I physically can't due to a disabilitating medical condition!
My dad was a mechanic (RIP) and when I came to him to fix my rigs, he would get out his lawn chair and tell me to bring him some corona. He would instruct while I did the work. He said he was determined to make sure his daughter could tell ya what's wrong and how to fix it. I have walked into many shops with my vehicle and had to call them out on their BS! WITH THE RESULT OF A DISCOUNTED REPAIR JUST TO KEEP ME AS A CUSTOMER! I might be a girl, but I ain't no dummy! There is no such thing as alternator fluid and you won't see me trying to check the headlight hoses!
Back to the point...if it's valuable to your heart then sink the money into it, but don't do just the head. Swap the entire motor or rebuild. Parts for the expo are cheap and typically simple repairs. Good luck!