That is a great price on a 350 gallon fully functioning set-up! You will need to find out what equipment it comes with, lighting, skimmers, etc before you can plan what to do with it. See if you can list the equipment here - size, brand, etc and we can help you out. A big tank is very nice to have, and if you can afford it is definitely the way to go. I wish I had a 350 gallon tank, although my current 240 gallon is plenty enough to keep me busy. :)
Water change frequency and quantity will be influenced to some extent by the equipment that you have and what you are trying to keep. On a tank that big many people do not do as frequent of water changes as smaller tanks. If you have a calcium reactor, that will help keep the water parameters better. SPS require better water than other corals and fish, so more frequent water changes may be needed if you are keeping sps.
Best of luck as you dive into this hobby. Nothing good happens fast , but many bad things do happen fast. The best advice I can give you is take your time, and ask lots of questions. I have been doing this for 36 years now, and I still learn new things every day.