If you want decent ride quality, primarily look for something that's specified with smaller wheels and bigger tyres. The overall wheel size of any car remains a constant but you can often choose between 17" wheels and low profile tyres, and 15" wheels with big fat tyres. The latter looks worse but rides much better. Avoid anything with sporty pretensions, e.g. Audi S Line, as the suspension tends to be firm and crashy. To go the other way you can look for luxury or comfort spec packages, or just a manufacturer that's traditionally been inclined towards better ride, e.g. Citroen.
If it's not driven much, then for cost and reliability reasons, you should get a petrol, probably normally aspirated rather than turbo, or possibly a hybrid but not likely at that age/price. Since it's low mileage, who really cares about fuel economy.
At that age of car, reliability mostly comes from two things: knowing and possibly avoiding model-specific faults and foibles, and getting good support from a local mechanic who ideally specialises in that marque or model. IMO peak reliability and durability was at about the age you're looking at, early 2000s, where the mechanicals and rustproofing etc were pretty sorted but electronic and environmental complexity hadn't got so serious yet. As ever there are exceptions, so e.g. don't buy a Mercedes from that time.