Well, that figure equates to $40K USD which is HUGE money for any RR/B of this vintage. Hell, you can buy Turbo Rs all day long for less than half that, at least here in the US, and even cheaper in England. Either way, the cars themselves are great stonking fun, very reliable IF maintained properly, drink fuel like nobody's business, but who gives a sh*t when you're enjoying such a fine automobile. DIY stuff is possible with them, and from what I understand, you guys down there in Oz are a pretty dedicated bunch, based on the activity on the RREC site, etc...
Most important thing is to pay someone knowledgeable to just give the car a look-over. It'll be the best couple hundred bucks you'll spend. I made that mistake long, long ago (of not doing it!) on a '96 Turbo R and it wound up sucking up $12K of my money over the next year to make things right....
Also, give a Turbo R serious consideration too

I'm on my third, a '90 model, and it's the best one by far. Better than the early (20,000 series) and better than the late (50,000 series) cars (of which I had both). The current car, just like what you are considering, is a 30,000 series, so it'll have the Active Ride suspension (which can be costly) but still be shod with the taller tires, so ride compliance is better than on later cars, which are crashy over bumps and early cars which are a bit to floaty for my liking. Granted, I've only ever driven one Mulsanne S, an '89, but unless you go for a Turbo R, you may as well get a Double-R instead...
Cheers,
Aaron