hi freeman. i would suggest you take partucular care with the bracket used to hold the float switches to the tank. many times, this type of set-up will fail, due to a loose float switch. so, make doubly sure that you have a very positive method of installation for these switches. i do not like to use a plastic bracket with nylon screws. these will not last long, in good shape, imo. plan these brackets properly, and take some time making them. they are VERY important to your set-up, and will make the difference between water all over the floor or not.
there are a couple of types of these float switches available, that i know of. the mercury type is the one which will last a long time, and operate many times without failure. they have 'motor rated' contacts. there is also a reed type. the reed style switch was devised to operate very quickly, (100 times per sec?) but not designed to handle a large load. while a 10 va solenoid is a very small load, i find the mercury switch to be much more reliable, and would be the correct switch for your application. they are the same price.
there are relays made with neon lights on them, to show when they are activated. there are also relays with a manual push feature, which allows you to operate them manually. these are also great features to have when troubleshooting your system, or simply to see what is going on electrically at a glance when setting things up. i can buy a relay (ice cube style) as i've described for about 10 bucks, so it is not an issue of price, but knowing they are out there. i rarely buy 8 pin relays (2 sets of contacts) when the 11 pin (3 sets of contacts) are readily available. you never know when the other set of contacts wil come in handy.
ensure all wiring is tight, secure and in metal enclosures. i have seen a few of these types of installations, with simply wires thrown everywhere. make sure your wiring is neat. this will be a big help later, when figuring out which wire is which (numbering them is really great too) and will make for a better job in the long run. set things up, fool with your connections with wires nice and long, then once everything is the way you want it, trim them back and neaten them up, tie-wraping in place.
i have heard of water sensors, which operate a switch when water is present. i would strongly advise you to get one, and place it on the floor under the tank. when water is detected, both solenoids should shut off, and maybe an alarm would sound.
these are a few helpful suggestions, but you look like you got things covered. good luck.
one thing i really like about your project is, that it is only 'semi automatic'. meaning, to start the cycle, user input is required- you need to start the timer yourself to start the water change. this is something i would do also, i don't trust having this type of thing working when i'm not there.
hth, rick | 
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