First, introduce 1 10m optical fiber into the computer room (or two 4m adsl). The router should have two wan ports, and the switching capacity is about 7g. Such a router can superimpose two broadband, for example, two 4m adsl can be superimposed into 8m bandwidth, so that if one line fails, the other can be used as a backup, and at the same time, it can save money economically, at least with less money to achieve high performance.
A core switch needs a backplane bandwidth of about 1t, and four 48-port switches need a backplane bandwidth of about 8m.
If the optical fiber scheme is adopted, the core switch and the access switch will form a star-shaped core network with the core switch as the center and the access switch as the periphery, and the hosts on other floors will access the access switch in the sub-center nearby. Alternatively, a six-layer core network can be used instead of an optical fiber solution.
As for the gateway, it is not a problem at all, and it is generally easy to do. The simplest thing is that all hosts can automatically obtain ip configuration by using the same network segment, so it is better to divide multiple subnets. But the problem is not big.
The landlord's requirements are not detailed, the needs are not clear, and the changes now and in the future are not clear. It is not easy to guess, please be clear.