Looking at integrating phones & internet in a couple of sites but dithering on the hardware selection!

The adsl side of 2820 series (&2830?) seems to be a very high quality modem, but I can't find any reference to whether it is the same adsl modem used in the "cheaper" 2710 series - anyone know?

1 site has a single line & a 2nd site has 2 analogue lines - on both we need adsl access + wi-fi + cordless phones. The 2710 looks like the correct hardware but anyone able to give real-life feedback?

3rd site is a bit more complex currently uses a PBX with 2 BRI isdn lines (ie 4 channels) and a separate analogue line for adsl. The existing cat5 network is linked to a hub which is inside the network, behind a ISA Server so we don't want to link the existing network/hub into the ippbx for security reasons. Question is how to benefit from ippbx in this case? - I think the answer is to have a 2nd distinct cat5 network/hub that the ippbx connects to? - but to the ip phones need to integrate in any way with desktop pc's? Sorry if that bit is a bit vague - trying to get mind around the general concepts of ippbx's to start with! Once the 3rd site is set up, one of its important features is that it must be able to transfer calls from Site 3 to either of site 1 or 2 in a manner that is low cost & invisible to the incoming caller - is this something that VOIP on an ippbx can do?
Thanks