Everyone's comments on using Google Apps for Domains rather than Exchange is spot-on. The ten bucks a year you spend on a domain (the Google Apps part is free unless you go Premier, but that can be worth it too) takes a huge headache off your hands. Admittedly, Exchange can be a useful tool, but for six workstations, running Exchange is like trying to exterminate ants with an M1 Abrams.
Pros of using your current box for a server:
-Uses inexpensive, existing hardware
-Performance will probably be good enough
Cons of using your existing hardware
-Extremely limited if the server is a mission critical part
At minimum, you'd be wanting a PCIe hardware RAID card, but that still only buys you one thing. Even a basic purpose-built server, done properly through a vendor like Dell, can get you the following:
-Customizable warranty support for 1-5 years, including 24-hour part replacement times (or less) to reduce downtime - Note: This is HUGE, unless you'd like to have a spare ASUS mainboard on hand in case your current proposed server dies, since reloading your server OS and apps/files on different hardware bites hard
-ECC Memory (the alternative is keeping spare RAM around for your already-built box in case something happens, but ECC = better from an uptime perspective)
-Built-in RAID controller and enterprise-level hard drives
-Option for dual-redundant power supplies (in case your server is really mission critical)
-Built-in hardware-based remote access (What if the server freezes and no-one is there to restart it? Or, how do you know when a drive fails in your RAID array? a remote access controller can e-mail you in the event of a fault)
Either way, you'll also want an Uninterruptable Power Supply as well. Not a pro or a con to either side, just an observation. Also, either an offline backup service (e.g., Carbonite, Mozy, etc.) or minimally an external backup drive is a must, as RAID only provides data redundancy --it helps nothing if your client deletes a file which is simultaneously removed from all RAID-member disks.
In the end, your customer needs to decide what acceptable down-time is in case of a hardware fault, and that can drive your decision. I've run Server on desktop hardware successfully, but it is a real compromise, and your customer needs to know that and decide what is worth more: their time, or their data, so they can sign off on their decision knowing you've explained every available option.
i7-4790K @4.2GHz, GIGABYTE Z97X-UD5H-BK, 16GB GSkill RipJaws PC1866
Corsair 650D, Seasonic 1Kw Platinum PSU
Samsung 850 EVO 500GB, WD Raptor 600GB, WD Black 1TB, NEC 7200 DVDRW
ASUS STRIX GTX 980, X-Fi Titanium, Dell 2408WFP