What causes intermittent failure of a 2004 XKR convertable with the 4.

I just purchased a 2004 XKR convertable with the 4.2 litre supercharged engine and 78,000k on the odometer. When the engine fired up for the first time in the showroom, there was a brief few seconds of heavy metalic taping (like dry lifters) and some blue smoke from the exhaust. After a few seconds the sound and smoke went away. The vehicle ran fine during the half hour test drive after that and no more smoke or taping sound was noticeable. The dealer says that the car has been sitting for 6 months without being run at all and thus the brief problems. I suspect there may be more to it than that such as bad lifter(s) and leaking valve guide seals. They are now going to do a '140 point inspection' and will correct whatever problems they find. I don't want to discover that I have to tear down the engine in a month or two to replace lifters. Ther are offering an aftermarket warrantee for $4,100. which would cover oil lubricated engine parts. I don't know if it would be wise to take that or hope eventual maintenance would be less than that over the three year life of the warrantee. The XKR is going to be used on an occasional good weather basis six months of the year. I probably won't accumulate more than 10 or 12k per year on it. Do the problems sound serious and should I pay for this rather expensive warrantee or hope whatever repairs are necessary over the next three years won't exceed that cost? Thanks.