Considering the size of the vehcile you are driving now, you are obviously not interested in the small SUVs and want something bigger.
So, for a used mid sized SUV there are a number of decent ones out there. I would start with a used Toyota Highlander which is a better vehicle than the Toyota 4Runner. A 2008 Toyota Highlander in my area with 75,000 miles can be had for $12,000 which is well under your budget limit. A 2013 is worth $15,500 with 76,000 on it although it is a base model. But if you like the 4Runner then by all means go with that - they are reliable.
Don't want Toyota? Consider the Kia Sorento or Hyundai Santa Fe. Not quite as reliable as Toyota, they are still good vehicles. A 2013 Hyundai Santa Fe Sport with 41,000 goes for $15,500. So with these you will get a newer car with less mileage for the same money as the Toyota.
I owned a Buick Enclave and it did not give me trouble over 6 years and 70,000 miles. A recalled water pump was all that needed to be replaced over those 6 years. It drives like a big car which is pretty impressive for something as large as it is. However, as reliable as it was for me I have heard after you hit 100,000 or so the vehicles don't hold up so well from GM. They have cars that can do this, but the Enclave is not one of them. So if you can find a newer one in your price range then by all means consider it - but be aware after about 100,000 miles some major components can start to go wrong for you. The same holds true for the Denali. It is a great car for about 100,000 or so but after that point it tends to be less reliable than Toyota, Hyundai, and Kia.
Best of luck.