Voip Options

I have an existing client for web development. They have a single office in the UK but regularly call a number of clients overseas (all clients are in one geographic area / phone area code).
I am thinking that a VOIP network would offer them significant call savings. My current thinking is for myco to set up a VOIP connection using them as the main client but this would enable me to on sell any spare capacity / out of office time to other customers.

As a company we already have computers / servers and asdl connectivity and a relationship with an isp at the other end but have limited experience on the telephony front.

For start up we would be looking at 30 lines on an ISDN line (E1 ?).
As far as I can tell there would appear to be 2 methods for VOIP….

1) Buy a VOIP gateway that automatically recieves the analogue call, compresses it and routes via IP
2) Buy some sort of telephony card that gets the information onto a computer and run / write some software that handles the compression / routing.

I would appreciate any thoughts you guys have on the best approach given we have finite financial resources but a fiarly broad range of programming skills.

Any information on the hardware that would be required / likely costs of this kit would be useful

As the name implies simple answers would be appreciated.

regards