how much it’d cost to fit them.
Doesn’t cost much. Buy wheels and tyres together, then swap them yourself. Initial outlay is more of course but each set lasts twice as long so you’re not really out of pocket at all in the long run.
Some of the cars posted here would cost you an arm and a leg in fuel, and servicing/parts.
I would go for a slightly newer Passat/Mondeo/Accord/Avensis, whichever you prefer. The petrol heads will call them boring, but we are talking about motorway miles, the only sensible way to make motorways interesting is to put the radio on and chill out imo. So you want quietness and comfort.
My neighbour bought a 55 plate 530d a couple of years ago for ten grand, and it’s a lovely car. Another colleague bought a high mileage A6 for the same price – that was very quiet but the ride was rather firm, possibly due to it being the eco label.