I wouldn't trust an omega to be reliable enough for a business to rely on it, low mileage or not.
Why not? Many of them were originally bought by companies for that exact usage. Maybe Amba will comment on this, as he used one until about a year ago.
Because the newest ones are over 20 years old. Thirsty, complicated and unreliable. You can say different, but I certainly wouldn't trust one to be used on lengthy journeys every day.
Your diesel Astra is far more complicated than any Omega. My - fading - experience of breakdowns is that modern cars are no more or less reliable than older ones, but that the problems tend to be much bigger with commensurate increased costs/difficulty/time to fix them. Some culprits are wet timing belts, inadequate timing chains, high pressure fuel pumps, plastic cooling pipes, DI fuel injectors, poorly sited ECUs full of water, sealed for life drive train components, etc etc.
The key to any car being reliable is frequent use, and dealing with issues immediately. That means treating OE maintenance schedules and times as maximums and actually fixing problems instead of symptoms.
A lot of the reported unreliability of cars kept as toys - sports cars, classics, ultra-low mileage, fancy convertibles etc - is caused by their lack of use. They frequently don't start, run badly, leak or have bizarre intermittent faults. And because they're not used much, those faults are never sorted even though they could be at no cost in a minimal amount of time.
Some of those faults are fundamental but easily fixed: tyres well into their second decade; fluid leaks; brake fluid the colour and consistency of oxtail soup; worn out wiring combined with ancient, shoddy repairs; fuel systems contaminated with rust, water and other nasties; utterly oppsed suspensions; damp interiors. Some of them are because the owner does all his own work, but is both incompetent and only ever patches things up instead of replacing all the worn parts in one go - this is
very common with brakes
There is no reason why
a forty year-old any car cannot be relied on to drive across the country on a whim. That has been a requirement for any that I've owned or had anything to do with.