G'Day
I purchased a 2005 (MY 2006) RRS 4.4lt V8 late last year. The vehicle is fantastic. I love it. But, just as with any car, there are some things to look out for.
First and most important I suspect is a solid Service History and general condition. I brought mine with 165k on the clock and a checkered service history. I have spent a fair amount of time and money bringing it up to scratch service-wise. The largest expenses so far have been a new EAS Compressor, a new alternator, and a set of 285/60 R18 Cooper Zeon LTZ's.
IMO, Most Hitachi built EAS Compressors will fail at some stage. Same for the Denso alternator. The good news with the compressor is that LandRover has gone to a new supplier (AMK) and the new ones are much better.
Other smaller things that may need repairing/replacing on the earlier RRS are the Brake Light Switch, and the Diff Breather end. There is a TSB for the diff breather and an updated part. (
RRSPORT.CO.UK • View topic - How to change front diff breather valve/filter/vent)
I also had the Bluetooth issue where I had to put in the pairing code on my phone (iPhone4) each time I started the car. I got a replacement unit from Bodsy on the Disco3 forums (also via 
Bluetooth Exchange) and it's now all good.
I have recently had more Suspension Fault lights coming up randomly. But I managed to trace the issue to a dodgy left-rear EAS sensor. I found the issue by using the 4Dcan software from Storey Wilson (
Range Rover - Sport) and a cheap ELM327 ODBII bluetooth adapter from eBay.
I am currently investigating the cause of the infamous driveline shunt at low speeds. ie; slowing down at an intersection, then accelerating away before you come to a complete stop causes a delay in acceleration, and a rather large shunt in the driveline as the auto tranny kicks into gear. I suspect a software issue.
Also on the list is a very random P0441 error. Apparently a small leak in the fuel vapour purge control system.
I recently had a day out in the mud around Narbethong (Vic) with a mate in his VW Amarok. I used to own a Toyota 4Runner, but my RRS is 10 times better in the dirt than the old 4Runner. My mate was very impressed with the RRS as well. He's stopped calling it a pimp mobile now.
I love the power and sound of the V8. I'm not too concerned about the fuel bills at this stage. But I can tell you that there is a marked difference in consumption between City and Highway driving.
I hope this helps.