The International Plumbing Code does not specifically require a thermal expansion tank or other device be installed with a tankless water heater. However, if a storage tank is used in conjunction with a tankless water heater in a closed system, which is sometimes the case with re-circulation, then a means of controlling thermal expansion must be provided.
Yes, You should insulate your crawl space for the simplest reason: your crawl space loses a ton of heat and costs you a lot of money.
Tankless water heaters heat water directly without the use of a storage tank. When a hot water tap is turned on, cold water travels through a pipe into the unit. Either a gas burner or an electric element heats the water. As a result, tankless water heaters deliver a constant supply of hot water. You don't need to wait for a storage tank to fill up with enough hot water. However, a tankless water heater's output limits the flow rate.
To find out if you have enough attic insulation for the TX weather, measure the thickness of the insulation. If it is less than R-22 (10 inches of fiberglass 6 inches of cellulose), you could probably benefit by adding more. Most U.S. homes should have between R-30 and R-60 insulation in the attic. If your attic has enough insulation and your home still feels drafty and cold in the winter or too warm in the summer, chances are you need to add insulation to the exterior walls as well. This is a more expensive measure, but it may be worth the cost if you live in a very hot or cold climate. If you replace the exterior siding on your home, you should consider adding insulation at the same time.