The link below provides a handy 'tree selection' process. Hopefully, this will help select a tree that meets your requirements.

Here is a link that might be useful: Tree selection

The issue might be the watering bit. From everything I've read, almost any tree will need to be 'babied' for its first year in-ground until it becomes established. Especially for the first few weeks. Planting one 'now', you might be able to get away without providing extra irrigation as it is our wet season. So you can cross your fingers and hope mother nature keeps it adequately watered. But if we get a dry spell...

As overused as they are, Crape Myrtle is what springs to mind for me. But when you go to a nursery looking for one, make sure you are getting a tree form and not one of the clumping shrub forms. And yes, they are actual 'tree' Myrtles and not shrub Myrles that can be trained into trees. The following article has a table of the different varieties of Crape Myrtles and what kind of forms they grow in. Some might argue that Crape Myrtles are not low-maintenance since you 'have to clip off all the spent flower pods', but I would argue that you don't HAVE to. Many people don't trim these off and their trees still look perfectly fine.

Here is a link that might be useful: Crape Myrtles

Various manufacturers sell tree watering bags that slowly release the water and keep the root zone moist. You only fill the bag once a week. Also protects against sun scald and critters that might find your tree's bark tasty.

This might be a solution for the watering requirement, though.

Why not get a nice crape myrtle? We are in the PRIME zone for them - they need absolutely nothing to look beautiful - they are a no-brainer in Florida. Get a tree, in a variety that grows to the size you said you wanted, and it will bloom every year right around the time your father-in-law passed away. It would be so nice in his memory. There are so many nice colors from which to choose. Natchez (white), Muskogee (pale purple), Miami (hot pink) or Tuscarora (watermelon red-pink) all have good tree forms. See link below and pay attention to size and shape.

Carol in Jacksonville

Here is a link that might be useful: Crape Myrtle varieties by height

I was also thinking of crape myrtle. You need something tough because a dead or sickly memorial tree is a terrible sight. I have a Natchez and I like it a lot. It grows into a tree shape with very minimal pruning required.

The Arbor Day Foundation has a wonderful program for planting trees as a memorial for a deceased person. They will plant trees in an American forest and send a card in the name of the giver. Here's the website-
http://www.arborday.org/shopping/tictim/TIMSample.cfm

I learned years ago that we shouldn't plant trees for our benefit, but for the benefit of generations that will come after us. If you want to plant a memorial tree on your site, there is none better for your area than the Southern Live Oak (quercus virginiana), which has an average lifespan of 500 years. It will become a majestic and magnificent tree, a fitting tribute to your loved one. The Angel Oak in Charleston, SC survived Hurricane Hugo and is estimated to be 1,500 years old.