I used to deliver vehicles for a living and drove everything from battered old Transit vans through to brand new, top-of-the range BMW's, Jaguars, etc. Your idea of comfort might not tie in with mine but, for what it's worth, here goes:
The most comfortable cars, albeit with a fairly high price-tag, I ever drove would have to be any of the Audi A-series (A4, A6, etc). Alfa Romeo was another manufacturer who I always rated high in the comfort stakes as well (although their low seats might not be right for your back problem). Other names with fairly high prices that I liked were BMW, Jaguar and (especially) Saab. If I was looking to buy something from that list (but I wanted to keep the purchase and running costs down) I'd opt for a 2nd hand Audi A4. (Not too expensive and decent fuel economy but, like everything in that list, spares and servicing don't come cheap).
If my budget was lower, and comfort was the main criterion, I'd be looking to buy a 2nd hand Rover. (75 model, if the budget stretches. 45 or 25 otherwise). I know that Rovers have hardly been popular cars - which is why they went bust - but for comfort they're hard to beat. (I often picked up Rover cars from the Lotus car factory in Norfolk - Lotus used Rovers for their company cars and drove them to a depot near Dudley. I never wanted to get out of the car at the other end - I felt so comfortable!).
Just for the record: The most uncomfortable cars I ever drove were the Suzuki Swift and (worst of all, by far) the Subaru Impreza.
Hoping this helps,
Chris