The BMW 320 is good car, but for similar or less monthly loan payments you could get a really special car that's just a few years older?
If you choose well depreciation won't be much of an issue and if you only do £6k miles a year then your fuel and servicing bills will be small even with a fuel guzzling sports car. Plus you'll own the thing so won't have to worry about going over your mileage allowance (if circumstances change).
People get over worried about big bills on older cars. The reality is though that most cars these days should easily do well over 100k miles without any serious issues. To say otherwise is complete scaremongering.
I've never owned a car that's newer than 4 years old and in 17 years of older car ownership (all performance cars) I've never once had an unexpected 'shock' bill or catastrophic failure. Yet the best thing is I'm not loosing £100's a month in depreciation. A friend traded in his 18month old 3 series last year that he'd had since new and he was down £11k in 18 months. That's a hit of £600 per month - owch!
I'd say go older, unless driving a NEW car is more important to you than driving a GREAT car, or the lease deal is absolutely out of this world.