Definately would say your looking in the right place as MINIs are just great (I'm sure everyone onb this site would back me on this!). If you want my opinion, and it is just my opinion I would definately stick with the generation 1 MINI, the Supercharged version, and not the new turbo one. Its not just the sound, but its also the fact that its supercharged. I mean how many people have a supercharged car (other than MINI Cooper S owners obviously). Its altogether more of a raw car too, in my opinion the new Cooper S is just a little too refined and not in the spirit of the classic MINI Cooper S. I think that the gen 1 Cooper S is more of a sports car than a hot hatch, but I'm sure someone will want to correct me on this

. But don't worry its not too raw, and my girlfriend loves my car, both to drive and to be a passenger (she likes her creature comforts). Not to mention you'll also be able to get an older gen 1 Cooper S for a lot less than the latest model.
As for the question of a hatch or convertible, well.... thats a tough one. I have a hatch, the reason being that:
A) I could not really stretch to the price of buying the convertible as they are more expensive generally.
B) for some reason it was more to insure the convertible (and being young this made it out of my price range).
If neither of these problems existed I would probably have gone for the convertible. The other thing to consider, and I think I'm right in saying this, is that they are still making the convertible Cooper S Supercharged, meaning that you could get a much newer gen 1 MINI Cooper S if you went down the road of getting a convertible model. They don't make the latest R56 model of MINI in a convertible yet, so if you went for a cabrio you'd have to get the supercharged model.
The body kit you spoke of is called the Aero Kit, and can be bought aftermarket from MINI for both the convertible and the hatch. I'm not sure about prices, but I'm pretty sure its nothing to do with the
JCW tuning kit, and this is more to so with the engine. The brakes are a seperate kit aswell, but again this can be aded by MINI I think. I hope you have a big overdraft thoughas all these bits cost a lot to get added on to the car! You might be best trying to find one already fitted with the
JCW bits.
I hope some of this helps
Mike