I had a 540i before I got the 840, and they're pretty damn nice motors for the money. Especially with the 6-speed gearbox (which mine had) it's plenty fast for most driving although it's certainly no sports car. Mine had huge mileage, and although it did show some signs of neglect was still generally holding up much better than my Omega.
The HIDs on my Omega work fine, it's just that the beam angle is far too low. It's a real PITA coming off a slip road in the dark cos by the time you can see that 90 degree bend you're almost on top of it! Fine if there are no other cars around allowing main beam to be used. The only other car I've owned with HID is my 840, and those have a significantly better range. The lights on the 540 weren't that great but being halogen they were adjustable beam so no problems with range. I seem to remember the Vectra CDX my dad owned years ago had HID and I thought the range on that was noticably poor too. One of the lights on that completely lost all height control meaning it pointed straight down - required complete replacement at a cost of £800 under warranty - typical VX!
I'd agree that if anything VX quality is a little better than the French stuff (that is generally really poor IMO) but I'm not convinced it's better than equivalent Fords. They're still very platicky inside but seem solid enough. I'd expect all the equivalent Fords to handle better across the range too; VX's front wheel drive cars are all pretty mediocre in this department whereas Ford have been getting better and better for a few generations now. The RWD cars are a different matter but neither of them really do RWD now other than niche models "borrowed" from the parent company (Monaro, VX220).
My Omega has had a fair bit spent on it (I have the receipts), including the best part of £1500 a few months before I bought it. Considering the value of the car isn't much higher than that I wouldn't say it's been completely neglected. Stuff like the door check strap just shouldn't break whether maintained or not - I've never seen another car with a hole punched in the metal of the door due to a seized check strap, and I've seen some seriously neglected cars in my time. Likewise the rust issue at the bottom of the doors is a bit of a joke for a late 90s car, especially considering the paint is in generally good condition. My sister's 1998 Laguna doesn't have any rust and I consider that to be a bit of a shed lol!
Alas, I had to tolerate a Ford Focus for 2 years. Worse handling car I've owned since my old Triumph Acclaim. Its build quality was poor, both from a design and manufacturing point of view. I've had a few Vx (4) and Fords (6). For a cheap, mass market manufacturer, their quality is higher that of their competitor (Ford).
The check strap should have been fixed (a simple service procedure, takes 2 mins and costs pence) before it got that bad.
If your HIDs are simply too low, there is a manual adjustment on them, but technically be callibrated afterwards via Tech2. It may be they just need calibrating in first place.
The high value of receipts you have to make me suspicious that it may not have been as cared for as it could have been, hence the high cost of maintenence all of a sudden (and probably reason it was sold). Some sellers are proud that "2k spent in last year" etc type claims - these claims put me off, as to me, things have been left until it has failed (and hence more expensive...)
As you say, I wouldn't spend £25k+ on this car, but not sure I'd be happy spending £25k on any other similar car either...
So, was the 540 (circa 1998) a v8?