I personally don't think it is too much of a tank, but you REALLY need to know what you are getting into (just beyond building it yourself).
First, I think 30" is a bit deep IMO
. You will have plenty of water volume going something like 24" deep but it will help with the lighting down the line.
Do you understand that weekly water changes with 400+ gallons is going to be around 40 or so gallons? That really is a lot. You need to have a place to make your RO
and mix up the salt. Salt will be running you a lot per month just to make up the PWC
Can the floor of your home support over 3400 lbs of weight? Not only will you have the weight of the water, but then you need to add in that tank which won't be light, plus the stand and all the equipment (plus a sump!).
Something to think about (seriously) is to make a sump first. You will need one for your tank anyway. See how that goes. You can even run it as a tank and put in some live stock even with sections and see how it goes. Then not only will you have some idea of the maintenance, but you'll also get experience building something that could eventually be out of sight so wouldn't need to be as perfect.
Just some thoughts...