What ill the network be for? Who will be using it? Is there any existing infrastructure you can use or need to connect to, or is this from the ground up? Will you need to have an internet connection? Saying between 25 and 500 computers per floor is really of little to no help. You need to get a more solid idea of how many users need to be supported. 25 users times 5 floors is only 125 computers. 500 per floor comes out to 2,500. Pretty big gap there, and it needs to be narrowed down to maybe a couple of hundred.
Where to put the server/servers physically? That doesn't really matter. They don't even need to be on the same property, as long as you connect them into the network somehow you are fine. What will they be used for? This is a key consideration. Do you have any existing servers that you can locate new ones with? Setting up a proper server is a lot more than just ploping the box down somwewhere. You need special electrical setups, fire supression, secured access physically, etc. If you can put it somewhere that is already setup properly as a serve room then you can save a lot of time and money.
Basic idea though would be a single router (or multiples if you want/need HSRP or the like) and then a switch closet on each floor. Each floor gets a switch that can handle the number of ports needed (see this is why you need to nail down a smaller range) and then that connects down into a central switch. The central switch then connects to a router and that to the internet. or if you have an existing network then you can lose the router and just connect the network to your central switch. You will need to consider things like VLANs, and how routing will be handeled too.
As far as wired vs. wireless, well it really depends. What type of computers are you going to be connecting? If you plan on having all laptops then wireless gives more freedom but security becomes a bigger problem. If all desktops then there is no reason for having wireless really. if it is a mix then you might want to consider doing both. There is nothing wrong with having a wireless network available in some areas and then wired in others.