I believe they do have 2 skins, but the bracket can also rust, and these are about £50 to replace I think. A new exhaust from MINI is over £300!! I've replaced mine with an after market PlayMini exhaust and it's great, much cheaper, all stainless steel so won't rust, and has a really nice sound. I appreciate you may not want to change the look/sound, so I think there are also exact copies of the MINI exhaust available too, that are much cheaper. You should be able to hear if the exhaust is blowing, and feel (don't actually touch it'll be hot!!) around the rust when it's running to see if any air is coming out. The exhaust will pop a little whe you take your foot off the gas, so this is normal.
If your worried see if they'll knock a bit off the price. Personally I wouldn't let it stop me buying the car if it's the one you want! Good luck.
