I've done some pretty long journeys in a big variety of cars, I'm not tall but I do have a bad back, and have done for 15 years. For example I drove coast to coast in America in 3 days, including one 1100 mile day. Yesterday I drove 500 miles. 2 years ago I drove to Puglia in Italy. These are just examples.
The biggest thing for me in terms of comfort are seats that aren't old and cruise control.
Older seats, no matter how plush they might have been when new, have sagged and no longer provide the support they originally did. Like a battered wingback leather chair that looks inviting but ultimately is terribly uncomfortable.
Cruise control allows you to rest your right leg, people who don't suffer with sciatica might never realise that constant slight variation in ankle angle and leg extension controlling the accelerator over long distance motorway driving can be very painful, especially triggering shooting nerve pain.
Yesterday I got into the car already in a bit of pain, but I got out at the end with no more pain than I started which is a really big deal. Especially if you have a kid and need to be active and involved the next day.
The other big factor for me, as someone with bad tinnitus and partial hearing loss, is sound deadening. Cars have a lot better sound proofing than vans and the resultant tinnitus aggravation from driving long distance in an older or cheaper car or van is really noticeable that evening and the next day.