The BMW warranty on the M3 is a joke over 60K. It costs way to much, has an excess, and has more exclusions than below 60K I believe. Some of the earlier E46 M3's are very tired too, and I can see some big bills on the horizon for some people if they do not buy with care. You still have the running costs of a £40K (or in modern terms £50K) car even if you only pay £15K for one.
I used the Warranty on my first car at around 40K when the Vanos failed, that was a £2200 repair, but covered by my warranty in full. I also used it for a number of little problems, such as broken bits of suspension, electronics glitches and other annoyances, though none major and all paid for. I bought both mine brand new, so imagine a car with 60K - 70K on the clock, you are likely to get into new clutches, new brakes, new diffs (or many if you are a drift nutter like someone I know

) and maybe even major engine work, which will cost big.
To be fair bm3w (or mtorque) is not littered with horror stories, so I think the E46 is a solid car, but as they get older the bills are likely to get bigger, and with a highly tuned sports saloon they can get very big if you are unlucky, making that £15k bargin a bit of a nightmare.
Buy with more care than you would a £15K hot hatch is all I say.