If I was you I think I'd just stick to hiring, it'll be a lot less hassle. Anything you buy will need to be taxed, insured (how easy would that be if you're not a resident of this country and it's business use??), MOT'd and serviced plus you have the potential for reliability issues, a flat battery if the car is left for long periods of time and so on.
Hiring may cost you more however it's pretty much complete peace of mind and zero hassle bar the booking of the vehicle. If you were to buy I certainly wouldn't be looking at the RR or the RS4, they'd do the job however the potential for big bills and running costs would make them a complete no-no.