I've actually got an R56
MCS and have had it since may '07 so I can give you an opinion of someone who has had the R56 "lived in" experience. In addition, i am in ClubMini so i get to see all the newer add-ins as they come along.
Firstly though, I'd like to discuss you're plan to buy a diesel. I think its a great idea, particularly because you are going to buy a diesel. The amount of torque quoted from the diesel engine should really push it along nicely. I have driven a petrol Cooper quite a few times and you really need to stir it up to get it mobile. No offence to Cooper owners but you have to remember I have hopped out of a
JCW MCS so i am looking for more torque. Personally I would be happy to own a Manual Cooper. I had one for a month whilst waiting for a part from Germany and i really grew on me. Good handling light and superbly cheap to run. So, in my opinion, Mini D could park in my driveway any day. Don't forget you can get those kits too that allow you to run it on old chip fat and other Bio Waste.
I think the other respondents have covered of most of the options, and i agree with Wizzz, RE: the Chilli Pack, its good value. The comfort access is simply keyless ignition. It seems expensive, but Geeez, I wish i had it. Its so damn Groovy! The sound system you get with the Chilli pack is fine. The sports seats are great, i have driven the cars with a few other seats and i think the sports seats are the best all-rounder as far as access to the care Vs support for the twisty bits.
As far as sports suspension goes. I had that on my
MCS and basically the sway bars are thicker, springs and shocks..... a slightly stiffer ride. I ended up putting in KW coilovers anyway and changing the rear sway bar for an even thicker one, so i guess that as you get more adventurous you could get better after-market stuff than what is being offered at a premium from mini.
Best of luck
S