Not owned an ST220, BUT, I have owned the same vintage of Mondeo, and a Jaguar S-type 3.0 V6, which has the same engine. Couple of thoughts:
- Engine is nice, not going to pull up any trees but moves along well enough. However I would guess it will be a pig to work on though. SWMBO had an x-type 2.5v6 (again, same engine, albeit smaller cc). Unless you're a double jointed proctologist, or have the hands of a 5 year old, det ready to lose the skin on your knuckles.
Second, I would take the car through a carwash and then check for damp in the rear foot wells, these had an issue with water not draining correctly from the doors (can't remember the cause, something to do with the wax they use to seal the panels and a problem with the plastic vapour barrier). The upshot is rusted rear door bottoms and damp carpets.
Normally, I'd say that the affected car would have rusted by now, but if it has genuinely been garaged, the symptoms may not be showing yet.
Beyond that, they handle pretty well and are generally a nice thing to cruise around in. I'd say go for it!