You said new, but I am making the assumption you meant "new to them" car. While I agree that ergonomics are personal, if you're looking for a place to start, you can try test driving the car we just got for my wife (who has 2 herniated discs in her lower back). We got a 2005 Honda civic with no issues, almost no rust, and 46,000 miles. We paid $7,500. I wouldn't go newer than 2005 for a civic as there were some engine issues starting with the 2006 models for a few years, and we won't spend the money for a recent model. I also wouldn't get one with more than 120k miles, or a ton of rust. (As those are just precursors to issues)
Civics are known to be fairly reliable, and it's probably one of the most comfortable vehicles she's ever driven. It also gets ~34mpg, which you could probably do better on simply by driving more conservatively than she does. If you get one with minimal rust and low miles it's going to cost a little bit, but the total ownership cost of civics is lower than a lot of other vehicles. Just last month I read a consumer reports article that put civics at #2 (I think) for cheapest lifetime ownership cost. If our current experience holds up I'd say we have found the sweet spot for vehicles (for her)... cost effective, not requiring much maintenance, reliable, comfortable, and easy to work on (which I haven't had to yet).
I don't know that this is a MMM recommended vehicle, but I am assuming your parents are going to drive more places than MMM himself does, which means maintenance items, especially ones that are mileage related vice age related, will be a higher factor for them than for him. This is just one idea, and may not be right for their situation, but it worked out well for us.
