Having just spent all morning repairing my stupid chlorinator, I'll point out a few things. The valves that are used on the plumbing controls are 24VAC and they can pull more amps than the arduino can provide. They are slow moving high torque devices that only use power when they are activated or you flip a switch on the bottom. They aren't cheap. Pools use large plumbing, mine uses 2" PVC pipe and thus, the valve are big too. By the way, the valves and actuators are two separate devices and the actuator mount on top of the valve with 4 stainless screws.
So, you're going to need a high current 24VAC transformer, mine is 10A and gets hot, however that could be because the chlorinator pulls 8A when it turns on.
PH sensors for the most part require replacement every year. Some of the very newest technology is better, but I don't know much about them. Same with the chlorine sensors. Since you mentioned salt, I'm assuming you're going to have a chlorinator. These things are often a lot of trouble to maintain, especially if you live where the water has a high mineral content. One thing they don't tell you at the dealer is that the chlorinators cause PH creep. That means the PH keeps going up over time and you have to keep adding acid to pull it back down. So, what you save on chlorine, you may wind up spending on acid. Also, calcium builds up on the plates inside the chlorinator and you have to clean them regularly. They don't tell you that either. The salt will actually cover the plate and the chlorinator will stop working. The manufacturers tell you to soak them in a dilute acid bath to clean them; DON'T DO THAT. The acid will accelerate the wear on the chlorinator and you'll need to replace that expensive item waaay too soon. We're talking from U$200 upwards to U$700 depending. Clean these things with a 1200psi power washer; you'll want one to clean the pool deck eventually anyway. The calcium buildup in the chlorinator will break off the plates and fly out into the pool causing a snow like residue on the bottom of the pool. If you have one of those pool sweeps that crawls around the bottom, it will pick it up for you. If you have one of the devices that swims, it'll just pile it up in the corners for you to scatter when you use the pool. If you have the jets in the bottom of the pool, same thing. They'll sweep the snow into the corners for you to come across later. This stuff will also plug up your filter so you have to clean it more often. If you keep the PH lower than recommended, you can mitigate this problem a lot. Try keeping it around 7.1 or so (I run mine at 7 most of the time. At 7.8 the pool will look like a snow bank in a few weeks.
However, even with all it's faults, I recommend a salt pool. It feels great and you don't have to keep chlorine bottles around all the time. You do have to have acid storage though. Remember, anything solid that dissolves in the pool has solids (duh) in it. These solids will be in your pool forever or until you backwash out enough water to remove some of them. Nice clean rainwater is your friend in this respect.
Many states have caused the big retailers to lower the concentration of chlorine in the stuff you buy. It used to be over 30%, now it's less than 15%. This is because the methamphetamine cooks use this stuff and they wanted to thwart them. However, it just winds up costing us regular people more money. Look to pool supply places and ask them the concentration when you buy it.
So, you'll need a relay for every control you want as well as an actuator for every valve you want to mess with remotely. I have 4 actuators and a couple of the valves are manual only. That's because there's not much need for switching between the pool returns and the in-ground cleaning system or changing the rate on the skimmer.
My recommendation is to start really small. Have the people installing the pool provide the absolute minimum you can get away with. That usually means a mechanical timer for the motor and a really simple chlorinator control. Get the valves you want right up front because working with 2" plumbing is a real hassle. You absolutely can add them over time, but it will mean some digging and replumbing of whatever you have. Then, when you have worked with it a while, add something like remote lights so the kids can turn them on at night when you're skinny-dipping with a lady. Then you can automate a valve to get a feel for it. It's not that hard, but the adjustments can get annoying. For example if you want a fountain to run slow sometimes and fast others, that's two valves, not one. You'll see what I mean as you go along. Oh, at around U$50 a valve and over a hundred for the actuator, you'll soon limit your expectations and stretch the project over time.
Do not automate the backwash. This is too prone to error and you could easily backwash a bunch of dirt right back into the pool, or your bedroom. Take the time to stretch that hose out to where it actually needs to go and then turn the valve. This valve is expensive also, nasty expensive. I had a neighbor automate it and flood his back yard. The chemicals didn't really hurt anything, but it was a muddy mess for a while. Where I live we have to have water delivered for a pool and it was two truck loads to get him back to normal. Those two inch pipes and multi-horsepower pumps can move a lot of water in a small amount of time. I laughed a lot at his misfortune until I got black mold and it was his turn.
I haven't even started on solar heating and the perils of putting a few hundred pounds of pipe filled with water on your roof. Nor have I mentioned the calcium build up on waterfalls or other features that has to be cleaned off using acid and a stainless brush. Things like getting an accurate read on the actual salt level in your pool when the chlorinator control is telling you that there is 1900PPM when you know for a fact that there is over 3000ppm in there can be a challenge.
Scare you off yet?