psicochaz,
Security should always be your number one concern, regardless of the size of your organization.
NAT (Network Address Translation) is not a security mechanism per se. It deals with using private addresses as per RFC 1918 within your organization and then translating some or all of those addresses to valid Internet Addresses.
You state that you have about 50 users accessing Internet and E-mail. This alone should be enough to warrant having a firewall as your users may potentially be hacked, backdoor'd or receive a Trojan. You also state that your only outside connection is your Terminal Service. If you ask some of those script kiddies, I'm sure there may be more outside connections that you may not be aware of.
Allthis said, on the low end of the spectrum I suggest deploying a personal firewall to all your clients and servers. On the other hand I would also suggest (budget allowing) to deploy either a software based firewall (something like Checkpoint Firewall-1)or go with a hardware solution. The optimal solution IMO is to find an old 386 or better, add some RAM and at least 2 NIC's and install any flavor of Linux and run IPtables. This last option is probably the cheapest and best solution (having donethis myself for some customers), but may have a steep learning curve and initial high maintenance (while you learn if you don't know Linux). Once you are comfortable with Linux, it's no different or harder than maintaining any other OS.
Last butnot least, remember that Network security doesn't only work to protect us from the outsiders; it should also cover protection from those on the inside as well.
