Congrats on your possible move, I hope it goes well.
I'm in FL, so sand is the typical footing down here, readily available and decent price. If you go with sand, it should be about 4 inches deep, any deeper and it's hard to move in, any less and it might not provide enough cushion. Down here lots of people are tearing out their footing with rubber in it as it gets too hot and has to be replaced frequently as it 'migrates' away from the arena. As a note to the sand make sure it is angular sand, not round, round sand will cause the horse to slip and slide.
You'll need a good base, That will be determined by the type of soil you have and how well it drains. You will need someone with heavy machinery to level, grade and compact the base for you. After this is done you could DIY the footing yourself, but it'll take a bit of work and time. The base should be about 6 inches down from the surface, you may need crushed stone or rock no more than 1/4 to 3/4 inches in size, and have that compacted as well. Make sure you allow for a drain, and have the arena sloped a tiny bit, crowned in the center to help the water drain.
As for watering, sand doesn't take to much to keep it going, but too little will make it dusty. You could consider a watering tank that hooks up to your ATV and then dump the unused water from the horses water buckets or troughs into the tank (strain it first though to prevent clogs) and use that to water with, it conserves water which is always a good thing.
As for dragging the arena, this will vary a bit, some people just drag chain link behind their ATV's, some spend a couple of thousand dollars on a arena machine. Then there are those who will hand rake an arena of that size to level out any deep or shallow spots.
For the barn I like having runs off the stalls and plenty of light and ventilation. For the aisle I like rubber pavers, but concrete is okay too. On the side opposite the stalls I'd have a tack/feed room combo, and maybe an indoor wash rack/ farrier bay. if the area you're in is dry I'd assume it's hot also, your farrier would appreciate a place to get out of the sun for a while. You can add lights for the winter if you run electricity.
For the hay, a hay barn is best, someone I know has a smaller shed near their barn where they put a few days to weeks worth of hay to keep it closer but out of the barn. I like that idea. I've also seen people buy multiple hay nets and fill those up for a whole weeks worth of feedings and hang them in their tack/feed room, and pull one down for morning/evening feeds.
One thing I really like in a barn is overhead fans to keep things cool in the summer. You can even use solar panels to run them if you know how to set them up.