VOIP implementation advise

Hi

We wish to implement VoIP to (primarily) communicate with overseas
office, but at the same time reduce the phone bill for the office in
general.

We have a 1 MB leased line connecting to a Cisco 2501 router (ISP
managed), then a Cisco PIX 506e, through to four 3COM 4288g manageable
network switches.

The remote side will be implementing Cisco Express.

A couple of questions -

i) Is it recommended we implement Cisco Express too, for compatibility
issues, or can we use any system (e.g. Avaya).
ii) We will outsource the implementation to external companies, but
wish to manage the maintenance. Given that none of the of the IT
department has
experience in either Cisco Express or Avaya etc, what is the most
straighforward application to learn.
iii) One method of implementation is to insert a VoIP card into our
existing Samsung phone switch and use QoS on our existing
infrastructure. Another would be to leave the existing infrastructure
for data only and use a SIP gateway and seperate ADSL line for voice
(which one company is advising). Any thoughts? Their claim is that QoS
can only go so far, and a seperate ADSL line/dedicated hardware is what
is needed.

Would be grateful for pointers.

Cheers