Your version of the V10 story is pretty much true but incomplete. There are also misconceptions. Let me try to bottom-line it for you.
1. When V10 came out, Ford "E" Series (vans, Class C's) and "F" Series (pickups and Class A's) got the same 275-HP Two-Valve (the classic One Intake Valve/One Exhaust Valve design). These engines had only a few threads holding the spark plugs in their aluminum heads, and a many had failures. This was 1997-98-and-99 Chassis years.
2. Come Chassis Year 2000, Ford re-designed the heads, labeled them "PI" for Performance Improved, went to 305-HP, and this included more plug threads. Problem now, some of the heads made at some of the component supplier plants had the threads and plug seating surfaces misaligned. This caused a few more failures. From what I understand, it took till Chassis Year 2004 for every head installed on a V10 to have correct spark plug alignment.
3. In the early 2000's, Ford upgraded the V10's going into F-Series (pickups and Class A chassis) to a Three-Valve 362-HP design. That left little room for spark plug holes so Ford came up with for very different spark plugs and a whole new series of problems came up. If you're looking at a Class C, NONE are affected by the latest set of issues because NO E-SERIES EVER GOT A THREE VALVE ENGINE!!! Don't let people freak you out over those problems.
If your Chassis Year (look at Ford's label in the door jamb and Owner's Manual in the Console) is 2000 or newer, I believe you are at little to no risk. As proof I offer my Signature. I knew about the spark plug issue, researched it, and decided owning one was a safe bet.
Hope this helps.
