The E46 M3 engine is pretty bullet proof assuming all the recalls have been done. There were problems with some of the early cars circa end of 2001, though mainly within the US. This was predominantly down to oil pump problems and there have been a couple of major recalls too around the bottom end, which a 2002 car will have been subject to, so you need to ensure that not only the servicing is up to scratch, which it seems to be, but that all the recalls have been done as well.
I have been a long time member of MTorque or BM3W, and have had 2 E46 M3’s from new, covering around 53K in the first and 44K in the second, so I have a very good fix on what it costs to run an M3. It is fair to say that of late there seem to be more and more posts from the later adopters indicating a growing list of problems, many small and no more than annoyances, but without a warranty they can be expensive. I had a Vanos fail on my first M3 and if I was paying for this myself it would have cost £2400 to put right. You should also note that BMW have recently reduced the cost of Warranties (I think) on the M cars, though they still have some clauses in there and excesses.
I would personally not put Conti tyres on an M3 either, and if the car has 19’s there are only 2 makes that you can fit which have the correct sizing of an M3 tyre, Conti is one and Michelin PS’s are the other. The PS2’s are a much better tyre than the Conti and the car runs much better on them, especially in standing water and damp conditions, and provides more steering feel and consistency too. The Conti’s are not as good in mine and virtually any M3 owner’s opinion who has driven on both.
If you get a good one it should cost you no more than tyres and servicing, but bare in mind that if you enjoy the car a set of rears may last 4000 miles, and for PS2's you are looking anywhere between £250 - £280 per rear corner. I have had roughly 10 BMW's and the M3's were a more expensive car to run than any of them.
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