Some of the basic things that were not considered when customers brought me on:
Are there remote employees? Will they need VPN access? What platforms are they using to connect? Can you verify that the endpoints are secure?
What is the anticipated volume of mail? In this day, it's often much cheaper to outsource to Google for smaller installations, but in some cases it makes a lot of sense to keep in-house.
When hosting your own web server how much downtime is acceptable? Do you need 24/7 uptime or will you have maintenance windows? What if your primary site burns to the ground? Do you have the floor space and adequate cooling? How much traffic is anticipated at the beginning of the project? How much do you expect to grow?
What applications do you need in-house? Accounting packages? Company intranet? Database? How will you separate your LAN for security purposes? Do you take credit cards as part of business?
What infrastructure applications do you need? Can you afford downtime on these? How many ports/switches do you need? Wireless? Separate backup LAN? OOB management for your servers?
Before you even start pricing hardware, find out what your customer needs and wants and willing to pay for.