Keep in mind that A) Blue book is a farce with such a low production number car with so many specific needs and B) These cars, at least on our shores, are basically $18K-$20K cars.
I say this because while you may be able to find a good deal out there, most of the cars in the low teens and I venture to say about every car in the sub-5-figure category will require a really good amount of work.
My car I have done valve adjustment, Inspection II, an additional oil change, and fan clutch. I need to do the brakes soon as well, which will be several hundred $. I also bought a set of 18" M-parallels for it. Mileage is 46,000 right now. I paid $15,500 for the car and put about $1,500 in for maintenance, $1,800 for the wheels, and $4,800 for a new bare metal respray [the original paint was knackered from Texas summers outdoors in sandstorms].
I know that the wheels and the paint were over and above necessities, but just keep in mind that if you aren't doing all your own wrenching, it will add up and needs can add up quickly. For a true, full books, no stories car I think you're looking at $18-$20K, or you can take a chance on a cheaper one. Just make sure you have some money to take care of unforseen problems or the ownership experience can take on a different tone [I see that you have an e39 M5 but I have no experience with maintenance needs on those, though I'm sure it ain't cheap

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Just wanted to let you know what I have noticed over the year that I have had my car, and wanted to levy expectations for you.