How do I built a network using Mac for (up to) 50 people?

Nothing in your post is remotely difficult to do - at least at face value. There are a lot of variables, though.

Let's start with the obvious one - platform. Are those PC users prepared to switch to Mac OS X? Or do you need to support a hybrid environment with both OSes in use? What about tablet/iPad support?

Given that you already have a Mac contingent I assume there's no issue with application support. Does that apply across the entire company though? Some applications (especially, for some reason, finance/accounting apps are heavily PC-focussed).

Once you cross the platform bridge, the next question is what do you want/expect to gain from the network. If users aren't networked now, what are the reasons for getting networked? Is there a specific application/benefit? or just something that you want to do 'because'?

Once you get a network, what services do you expect to provide to your users? Since you mention '... transfer over data' I assume that means some kind of central file server...? If so, what kind of files? how big? what kind of backup/redundancy plan do you need?

Central printing and scanning is usually pretty easy. Central fax is a little iffier. In most cases I'd suggest a third-party fax service rather than try to run your own fax server, but there are options.

Decisions around printing hinge on volume of work (how many pages do you expect to print per month), speed, page format (do you need large-format printing or is letter/legal paper large enough?).

The trickier issue to get right might be the wireless network. Depending on the size of your environment you may need to balance multiple base stations to provide coverage. This is easy to do, but hard to do really well.

So, you see, much of where to start is simply asking questions, capturing requirements and then finding the right solution. Of course, the missing question here is budget. If you're on a shoestring then your options (and, potentially, solutions) may be much more limited. Asking the right questions (and having the answers) makes it easier to cost-justify, though.

As for help, a great place to start would be http://consultants.apple.com/ where you should be able to find a local resource that can help you design and build your network.

There are probably as many different designs and design requirements and environments for this network as there are companies of this size — you've been tasked with forming an IT department, in simplest terms.

Beyond what Camelot has referenced, you're also taking over IP address and DNS name coordination, at network monitoring and troubleshooting, network security, probably also at Active Directory or Open Directory for shared authentication, at implementing some shared storage, probably bring-your-own-device (BYOD) and mobile device management tools (MDM) such as OS X Server Profile Manager, probably also at moving toward some added network and server redundancy for your gear — having the only network server or the main network link down can hurt productivity — and a whole host of other IT topics.

Probably at managing many devices and at imaging client device software, too. Managing the software that's loaded onto various devices, allowing you to easily reload and easily upgrade systems. At tools such as Munki, too.

You're likely going to be implementing access points (APs) for your wireless, and not WiFi routers, too. That to improve coverage, and to spread the load. Where you can't run wired connections.

Whether or not you're going to host your own mail and web services, or outsource those tasks, too.

You're also in the range where I'd be looking at managed switches and related hardware, and potentially at some of the newer "software-defined networking" products. At features and tools that would allow remote management of your networking gear.

Your local Apple Business Services folks might be worth a chat, but you're pretty quickly going to be learning IT and IP, or working with somebody else to help design and deploy and troubleshoot your network. None of this is particularly difficult stuff. Well (and as Camelot suggests), heavily-used WiFi or heavily-used network links can be a problem, if you don't have the budget for good gear and access to some monitoring and troubleshooting tools.

With OS X, the archives of the Mac Enterprise mailing list can be interesting reading.

Off all of this, collecting the current and future requirements are key. I'd also expect you'll be rolling out Active Directory or Open Directory for distributed authentication, DNS, coordinated DHCP, and various other bits here — if not all of these immediately, then fairly soon after the initial deployment. (Otherwise, the care and feeding of individual computing and networking boxes and of individual users usually starts consuming all available IT resources.)

Welcome to IT management! 😉