You will probably find it's a 316i with an M-tech bodykit (like the car in my sig pic) the M-tech kit often get's wrongly refered to as an M3 kit. It's highly unlikely to be an M3 kit on a 4 door (though not impossible as it has been done, but completley custom).
The only special edition of a 316i I can think of is the lux, it should be fitted with 6 button computer and rear headrests.
If it is an m-tech kitted car with all those options you mention it does indeed sound like a very nice car, 4 door m-tech kits are very rare.
And 140K High mileage??!! You must be kidding, the car is almost 15 years old!! The M40 engines suffer from premature camshaft wear, so if still on the original it will probably sound very tappety, but it's a relatively easy and not to expensive to replace.