[Perfectlap]

I will give this advice,

96-99 is essentially the same car.
00-04 is essentially the same car.
05-08 is essentially the same car.
09-11 is essentially the same car.

Look for a car with low mileage within each range up above. Anything with greater than 50K miles that you hope to buy without sinking a great deal of money into needs to be a car that has lived its whole life in a moderate weather zip code or has to come with a large file of non-deferred repairs. If someone is selling a car that "needs nothing" and was driven greater than 50K miles in a climate that sees extremes of heat and cold... walk away. Case in point my car was mostly garage kept, driven only on weekends but driven year-round in the Northeast. The first 65K miles were flawless. But from 70K miles to 100K it needed more in repairs than the resale price of the car itself.

The other thing, mileage alone does not tell the whole story. That's why high mileage cars with no history are problematic. You can have a low mileage car that was driven weekly ( a good thing) but only on short trips (a bad thing). You can have a moderate mileage car that was driven daily for at least an hour in So Cal (win, win, win). This is why the pre-purchase inspection is important, I would absolute include a leak down and compression test. The other issue is sticky lifters or collapsed lifters, according to our engine experts here, high mileage Boxsters and Carreras nearly all have issues in this area.

The IMS thing is overblown. Your engine is more likely to go down due to a water pump that was not replaced proactively. The general rule is every 4-5 years. A failing water pump will claim many more engines for each IMS failure. But it is true a dual row bearing is better to have as it has a longer service interval. But when a dual row bearing goes the damage is greater. From what I've read over the years it seems that this issue is due to oil starvation or oil contamination. Upgrading the bearing does not address the root cause of these, it just gives you a bearing that is more resistant to these maladies. Replacing the bearing on a car that has an unknown history is a good idea but so is replacing the water pump. The other thing to keep in mind is that a car without a good history may have other engine issues brewing, hence the importance of the leak down and compression test. Replacing the oil before taking ownership is not a bad idea either if the seller will agree that the sale will be final pending an examination of the pan for any metal.

As for values, I don't believe any publication has an accurate assessment of these because from what I've seen the Porsche market has experienced a recent bubble up in pricing across all sports car models. I suspect that with a weakening credit market in the next 12-18 months and possibly a mild recession that prices for the mass produced Porsches (Boxster/Cayman and 99+ 911's) are going to depreciate at a faster rate than we've since since 2009. There are simply too many water-cooled Porsches for sale right now without nearly a deep enough pool of buyers to take the risk on out-of-warranty cars. If you do buy a strongly urge you to buy from an enthusiast directly in the Porsche forums first. Used car lot Porsches should come with a deep discount but instead come with a middleman's surcharge.

The mileage on that 99 seems problematic to me. If the oil wasn't changed twice a year, or immediately before storage more so.
The engine needs to sit in fresh oil or needs to be drive long enough to burn off the build up from moisture and dirty oil. If its getting neither, not good.