Accessing specific computer(s) from internet

Hi

We have a single-domain Ethernet LAN. Access to internet goes through Linux
firewall.

One of our customers does want a couple of computers (production testing)
to be set up in our net:
they must have access to some programs in those computers from their
location (something like VPN - they must have the possibility to move files
between 2 networks, have remote access to desktop, etc.);
those computers must have access to internet (IE and some web-based
mail);
those computers mustn't see any other computers in our network, and they
mustn't be visible to any of our workstations (a different address-space and
the connection to LAN through a router would be best for them);
those computers have to be in same physical net as other computers;
on customer side, the solution must not require any specific hardware,
or any specific firewall settings (p.e. opening specific ports) - i.e it
must be something very easy to set up.

What'll be best solution (in general) for this setup?
Thanks in advance!


--
Arvi Laanemets
( My real mail address: arvi.laanemets<at>tarkon.ee )