Yes. You are free to buy one but you cannot register it for road use.
No, you can't register it and drive it on the roads. Yes you will have problems.
You have two options,
1. Import the car and have a RHD conversion done on it (be careful who does the job as some people are a bunch of hacks and safety is a issue)
2. Wait a bit, Ford will be bringing out RHD Mustangs to coincide with the 50th anniversary of the Mustang in 2014 or 2015 for the international RHD markets (UK, Oz, NZ, SA etc). These will be manufactured as RHD vehicles and will not be conversions.