Unless the bike's exceptionally cheap - and I suspect it won't be, being a dealer sale - I'd say to carry on looking, the market has plenty of reasonable-miles, early RTs at low prices. I suspect my own 30k-miles '06 bike would realistically sell for around £3k - how does that compare to the one you're looking at?
Having said that, the RT/GS/ST line can generally take high mileages as long as things are looked after properly. First on the list to check on an '05 bike would be the servo brakes, which it will have - Googling will tell you all you need to know, but the system is well-known for failing, leaving you with what BMW call 'residual braking' (think around 15% of normal stopping power....scary!) and a big bill to sort it (think £13/1400, there's no cheap fix other than to remove the system completely, leaving you with standard, non-ABS brakes). Then I'd look at suspension - ESA units (electric adjustment) can fail and should be checked through all settings, standard units should be checked as you would on any potential purchase. The rear drive unit should be checked for play in the bearings. Engine and gearbox should be OK, the engine's a fairly noisy unit, being air-cooled, and the dry clutch (as in a car) can make things sound even noisier - speaking of which, check the clutch for slip, as fitting a new one is a major job, requiring the bike, in effect, to be split in half to get at it. Look at the engine front cover for corrosion - not a catastrophe, a lot of boxers have it, but it's an eyesore and a pain to sort it properly.
There'll be other stuff to check, but only as you would on any bike you're thinking of buying.
Let us know how you get on.
Pete
