first, welcome to asterisk I assume by ‘trunk calls’ you mean long distance.
you want a few things (here is what they are called)
VoIP termination (outgoing calls). You want to use an internet telephony service provider (ITSP) to get cheap/free calls overseas using your Internet connection.
Direct Inward System Access (DISA), aka remote cell phone access. If you can get a call into Asterisk, either with an ITSP or an analog interface card, you are 99% of the way there. You set this feature up in the dialplan (extensions.conf)
Your hardware is fine. Asterisk uses very little CPU power unless you are either encoding to a low-bitrate codec (like G.729 or iLBC). Your system can easily support ~10 channels of telephony if it’s doing nothing but routing.
It is possible to use Asterisk to terminate calls to PSTN numbers (landlines/cellphones/etc). You won’t need any extra hardware to do this, if you do it with an ITSP. You will need an ITSP to do this, or use your current landline. Sadly, it’s usually not free. There are a number of ITSPs in the USA that offer unlimited calling to the USA and Canada for a flat monthly fee, there may be one like that where you live. There is a very large series of lists here: voip-info.org/wiki-VOIP+Service+Providers . Since you are using Asterisk, you will need either one from the B2B list or one from the other lists that supports BYOD (bring your own device, becuase you want to use Asterisk instead of their phone adapter).
For your sales people- you can setup (very cheaply) a DID (phone number) from an ITSP that will feed straight into Asterisk. The advantage of this is that more than one person can use it at a time.
If you want to connect * to your existing landline, you will want a Digium TDM400 series card with at least one FXO (red) module such as the TDM01B bundle.
Aside from interface boards for POTS/PRI/ISDN lines, you don’t need to purchase any additional hardware.
All of the features Asterisk offers are programmed in software, so if you want to change your system’s behavior, you can do it by editing the Asterisk configuration files instead of having to buy PBX feature modules.
also I suggest you try the book Asterisk: The Future of Telephony. It is available from O’Rilley or you can download it for free under the Creative Commons license from asteriskdocs.org . It provides a solid introduction to Asterisk’s capabilities and how to utilize them.
Hope that helps!