You may wish to consider more elements in your design:
1. Cable plant and distance. You did not mention if you are re-using existing wiring or installing new wiring -- copper and/or fiber-optic. Most wired and wireless ethernet has a practical radius of 200 to 300 feet. If your buildings or plant are exceptionally large, you may be required to add an additional wiring closet and switch just to stay within cable distance limits and/or use equipment with fiber-optic connectivity
2. Business function and how it affects data flows. What part of the business is going on in each building (and each floor, if possible). Are these the only sites for this business, or is this network part of a larger enterprise? For example, if the plant has computers used as machine controllers, the plant may have low data flows in and out. But if the plant collects data for quality control -- for example, capturing high-resolution images of every product manufactured -- the data flow and storage requirements may be quite high. If some functions are consolidated, such as manufacturing planning at one site, and sales at another site, there may be high data flows between the sites during the business day or after hours. How many mobile employees do you expect? Will a large number of employees with laptops requiring wireless connectivity show up at one location? Is the layout of the offices and plant fixed, or are frequent changes expected -- is wireless ethernet a better solution for part or all of the environment?
3. Business strategy and how it affects information technology choices. Do any of the sites require internet access? Will internet access be consolidated at one location (where security and other servers may be consolidated) or is it distributed to each site? Is the company expecting to use outsourced computing services located in the internet or across a dedicated connection -- this may include voice-over-IP. How will those services be accessed? Will the individual computers run an operating system with all required applications installed, or will a thin-client or virtual machine solution (Citrix, Xen, Windows Terminal Server, VMware) be used to consoldate applications on a few servers?
4. Redundancy, Availability, and Scalability. No business is static, and disasters happen. Is your network design ready for that? If you have a single router per floor, what happens when that router fails? If you have a single server per site, what happens when you must take down the server for maintenance? Will you have dedicated support personnel to be hands-and-eyes on each site, or will you outsource that support? Can you add elements to the design so disasters can be worked-around automatically, or with minimal intervention on-site or remotely? If you use wireless, how will you deal with interference and signal degradation? You'll find wireless ethernet does not scale as well as wired ethernet.
The last item can be explored with a simple drawing with simple boxes on each floor for your network equipment and lines connecting them. Simulate your clients by connecting one picture of a PC to your network equipment and add a note like "x 50" . Now draw an X through your failed network equipment and write a short note on how you deal with the failure; include estimates on how long it takes to recover from that failure.
This is a reason you will see network designs that appear to have high amount of redundancy -- two switches per wiring closet and/or switches with multiple redundant power-supplies, switches with removable blades or which can be stacked; dual-uplinks from one floor to another; active and standby VPN connections across the internet; uninterruptible power supplies to support equipment for at least 5-10 minutes to allow a graceful shutdown or reporting a loss of power before going dead.