Having an arena to ride in, whether it's indoor or outdoor, certainly facilitates training, and provides the structure that can be very helpful for riding specific exercises with purpose. If you plan to build an arena, there are many considerations - size, location, accessibility and of course, footing. There's a wide array of materials available for arenas, from the traditional sand to manufactured footings made from textiles, rubber and other materials. If you're thinking sand is the simplest and most economical way to go, keep in mind that even sand comes in many different types and it's worth your while to investigate what's available near you..
Many of the companies which produce manufactured footings will provide free samples and free estimates for their products. The most comprehensive resource I have found to provide information and guidance on all phases of putting in an arena is the little booklet "Underfoot: The United States Dressage Federation Guide to Arena Construction, Maintenance, & Repair" which is availabe to purchase through the USDF
Great reminder about the importance of good footing! It is such a huge consideration!