Of course, short of calling Mansory and saying "do your worst", you'd find it hard to limit yourself to just one. Buying both a Rolls-Royce Phantom and a Bentley Mulsanne would cost £570,000 if you couldn't be bothered to haggle, and would leave you enough left over for either a serious options blow out something sportier for the weekend. Alternatively you could buy four Mercedes-Maybach S600s, or six Audi S8 Pluses. You get the idea.
But if you really did have just a single parking space to spare then you could opt to spend the entire sum on this, the new Lagonda Taraf. Not the most rational of decisions, but one that would give you what's probably the rarest and most exclusive saloon on the planet.
When Aston first showed the Taraf last year it didn't tell us much about it; we didn't even know its full name to start. Initially sales were limited to the Middle East, but the collapsing oil price is clearly biting (and turning billionaires back into mere multi-millionaires) and ordering was quietly opened up to other territories late last year, including the UK. Both left- and right-hand drive versions are available, and apparently a couple have already been sold in Britain. Don't worry, though - although officially limited to 200 cars, production has to finish by the end of the year, meaning that only about 150 will be produced. And there's still time to get on the list.
Our drive is in the prototype version, on the roads around Aston's ancestral home in Newport Pagnell. This is the same car that was unveiled at the Dubai motor show last year, its Topaz Gold paint intended to be seen in desert sunshine rather than the English winter, but even with grey skies and rain falling it's an absolute show-stopper. Oh, and if it's not gold enough for you, Aston is also offering optional 24 carat badging.
It's certainly big. At 5.4 metres in length it's longer than a long-wheelbase S-Class, although shorter than a Mulsanne or Phantom. But its presence comes from more than just sheer size. It looks like a motor show concept wearing numberplates, with a front end that seems to be pretty much entirely grille, a vast bonnet and - thanks to the tall glasshouse and heavy tints at the back - a palpable air of menace. It's the sort of car an up-and-coming dictator might trade his Grosser Mercedes 600 in for.
Impressions of the interior vary depending on which door you use. Get into the front, as few other than professional drivers will, and you could have a pretty much identical experience for far less money. The Taraf shares the entire dashboard of the Rapide S saloon (and therefore also, pretty much, the outgoing DB9), has Aston's older square edged switchgear (most cars got soft-touch replacements last year) and even uses the same door cards. That's because, under the surface and its carbon fibre exterior bodywork, it sits on a stretched version of Aston's familiar VH architecture.
The short front doors and a further-back seating position means shoulders are right next to the B-pillars, limiting visibility. The high glassline and heavily raked windscreen increase the impression that, when you shut the door, you're driving a vast coupe.
Of course, those with seven hundred grand to stump on a saloon are considerably more likely to be climbing in the back, so it's no surprise to find that more effort has been spent there. The huge hump that separates the Rapide's rear seats has gone, in its place a relatively modest leather hillock that covers the rear-mounted transaxle. Although the seating position is low, there is impressive headroom and pretty much all of the 200mm wheelbase extension over the Rapide S has gone into extra legroom. The rear seats don't recline, and there isn't much in the way of gadgetry back here (an optional iPad based entertainment system isn't fitted to the prototype), but it's far more comfortable and spacious than sitting in the back of any Aston-built car since the wedgy 1970s Lagonda.
Many Tarafs will never be expected to travel quicker than the agonisingly slow grind of a Middle Eastern city, or at the modest pace of the motorcade that lets their rear seat occupants wave to a crowd. It deals with this sort of trundling well, with a softened-off throttle pedal making it easy to do a super-smooth start and the eight-speed autobox programmed to behave with appropriate deference when left in Drive. Low speed ride is good; the Taraf still sits on steel springs, but it feels pliant over broken surfaces. The 6.0-litre V12 is more muted than in any Aston, and even has a quiet start function that does without the cheeky blip when fired up.
But the fundamental Aston-ness is buried in a very shallow grave, and it's not hard to bring it back to life. The engine's relative lack of torque means that requests for proper acceleration lead to revs, and revs deliver a subdued version of the V12's familiar wail, and that encourages you to try harder; before you know it you've taken control of the gears with the behind-wheel paddles and the Taraf is north of 6,000rpm and still pulling hard.
While most pluto-barges only tolerate corners, big brake applications and anything else that isn't serene progress, the Lagonda seems - like a true British aristocrat - to relish a good thrashing. It drives like a bigger but only fractionally less agile version of the Rapide S, the steering feels lighter and the brake pedal slightly softer, but outright performance is pretty near identical; the use of carbon allows Aston to claim identical 1,995kg kerbweights for both cars.
It's when you slow down to cruising speed that the Taraf's case wanes. It's short on waft, without the bump-pillowing abilities of its air-sprung rivals, and with noticeably more road-roar and wind noise getting into the cabin. It feels like a sports car that's been turned into a limousine. It is.
So, full circle, who is out there with enough cash to buy a Lagonda and an equal determination not to make a substantial saving by buying one of its cheaper rivals instead? Aston admits that the Lagonda's appeal is very, very exclusive, meaning that those who do buy one are certain to get the bragging rights that come from near-uniqueness. For a certain type of determined-to-be-different squillionaire, that's probably appeal enough.
Its real role is to make with the Ferraro Rocher and act as an ambassador, to prepare us for Aston's plans to relaunch Lagonda as a separate brand with more than one model. If they can keep the design and the driving experience in those future Lagondas, while adding some more refinement and shaving a couple of hundred grand off the price, then they might be onto a winner.
Engine: 5,935cc V12
Transmission: 8-speed automatic, rear-wheel drive
Power (hp): 540@6,650rpm
Torque (lb ft): 465@5,500rpm
Top speed: "195+" mph