Funny you mention that Tim, I am thinking of doing the same! I have an 18i and it definitely doesn't sound like that video!! Maybe up close it does but from the drivers seat it doesn't.
I absolutely love the looks of the zed and the attention it gets but really wish it had more power which is why I'm thinking of a 135i...plus its more practical. Only other option is to upgrade to a 35i
I got 7.5k off my 18i so you should be able to get a decent deal if you are thinking of an 18i or 20i.
The bigger engine the more discount you should be able to get mainly because their 'RRP" is very over priced. Fully spec'd my 18i m sport was 33.5k & I paid 26k brand new.