Under $18k? For an SUV with legroom? Good luck, unless you are talking an old one.
I am sure there are larger SUVs with more legroom, and I don't know how cramped it really is at 6' 5"... but I have been in a 2014 Subaru Forester and my kids sat in the back. My son is 15 and a teenager, and about 5'9". He siad the back seat had way more legroom than my Xterra (which I think has a lot of legroom in the back seats). It also has a lot of head room. Which is just as important for tall folks.
They start around $23,000. And are AWD, so good in the snow. And the gas mileage is pretty alright, too... like 26/32 or something. It might be worth it for them to at least sit in one... otherwise, I wish you luck. Under $18k is very limiting for an SUV.
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I did a quick google search and the "average" age I found for a Honda Pilot at $18,000 or under was a 2008 model. Most are not 4 wheel drive. And most have in the neighborhood of 80k miles on them at that age... at least the ones I saw. There was one that came up in NM that was a 2009 model, with 4 x4, though. So maybe look at those if they are willing to go that old. Personally, I'd be hesitant to spend that amount on a vehicle (particularly a 4x4) that was 7 years old.
I wonder... how often do your very tall brothers expect to ride in this vehicle? If it is only on periodic vacations, perhaps your parents would be better served to rent something for the vacation driving, and BUY something that more closely suits their every day needs (without the limitations of tall back seat passenger comfort). It'll open up a lot more options for them in their price range.