The M45 has been sold with a rather powerful V8 engine making 340 horsepower in first generation models (2003 through 2005) and 335 in second generation models (2006 through present).
The M35 is sold with a V6 engine making 280 horsepower.
The M45 is sold as a rear wheel drive vehicle, with the M35 being sold as a front engine or all wheel drive car. The all wheel drive model is badged the M35x. It drives with rear wheel drive unless the car senses slip in those wheel, in which case it switches to all wheel drive.
The name of these cars might be changing, especially in Canada. BMW has filed a lawsuit because they believe the line causes confusion with its own M series cars.
The old M coupes are out there on the used market, although not common. The newer M35's and M45's are just finding their way on the used car market now, usually off lease. Check on eBay for some good deals.