That's what I was afraid of. An extra figure of about $2500 would surely kill the deal for me, if I were to go that route.
The chart is pretty interesting. I can also remember reading on here through the years that by '06 the problems were basically solved. Even though I see that it was on a downward trend even earlier. And it's nice to verify that.
The info here can vary depending on who you ask, but from what I remember, the head gasket/head bolt issue was virtually eliminated starting with the 2004.5 Northstars and up, all the way until the engine was discontinued in 2011. I'd go 2005+ if I were you, as I can see you are definitely cautious. They ended up replacing the revised 2000 head bolts (which were lengthened from 1999, but still had a fine thread pitch) with LS6 head bolts from the small block V8 family (longer AND had a much coarser thread pitch for superior grip), that did the trick.
The head bolt failures did drop off hard after the 2000 bolt lengthening, but that didn't "eliminate" the issue. You can find 2000-2004 Devilles and Sevilles all day long on craigslist with blown head gaskets for $1,000. But you can also find quality, low mileage 2000-2004 Devilles on there as well.
Combined with what bass and Sub said, I'd highly recommend a 2005 Deville DHS or DTS, depending on what suspension setup you prefer. The DTS, which was essentially the next gen Deville (2006-2011), had a very plastic-covered, sterile interior. The Deville interior was much more comfy and pleasing to the eye. I have a loaded 2005 Deville, and couldn't be happier. The Northstar runs perfectly and is the smoothest engine I've ever owned! Best of all, I have peace of mind.