I imagine that what I am about to describe can indeed be accomplished,
just don't know the last little bits of the details. Hopefully,
someone will have done this before and can point me in the right
direction...

Three locations, all within 1/4 mile of one another.

The main site is a small business, and we are putting in a video
surveillance system with DVR. This small business is blue collar, so
there is no internet connection here, and there is no main computer
here -- just the DVR (with embedded web server) and whatever routing or
network devices we install. Putting an internet connection at this
site is *not* an option.

The other two locations are the owners' homes, and both have home
access to the internet via cable modem. Both have a basic router /
switch w/firewall at the current time, and are open to upgrading that
routing box if necessary.

Owners are open to placing antennae at the business site and at home,
and I am thinking point to point wireless-G for each connection.

What I am trying to design is having each of the owners contact the
business via a wireless link, and be able to hit the DVR webserver
through a browser and not have to change their PC settings or router
settings each session. (They are new to the internet!)

I understand "cable replacement" access point pairs, and have deployed
them. I am rather conversant with the tenets and implementation of
straightforward networks. There is at least one piece of this,
however, that is not clicking for me.

If we go from the the first owner (site A) to the business (site Z), we
can do a wireless link. Same for A to Z. However, there are then 2
routers at site Z, and only one DVR/webserver. Additionally, how will
the other computers at site A and B be able to access the rest of the
internet?

For discussion's sake:
Site A: Comcast dynamic external IP
Internal gateway of 10.100.48.1
Netmask of 255.255.255.0
Running basic DHCP, but can assign fixed IPs if necessary; main PC
fixed IP of 10.100.48.9
Can handle a directional antenna pointing at Z.
Can have wireless access point on site, such as a WAP-11 or WRT54-G in
infrastructure mode / creating a wireless bridge to Z.
A-side end of bridge is 10.100.48.2 (?)

Site B: Comcast dynamic external IP
Internal gateway of 10.100.49.1
Netmask of 255.255.255.0
Running basic DHCP, but can assign fixed IPs if necessary; main PC
fixed IP of 10.100.49.9
Can handle a directional antenna pointing at Z.
Can have wireless access point on site, such as a WAP-11 or WRT54-G in
infrastructure mode / creating a wireless bridge to Z.
A-side end of bridge is 10.100.49.2 (?)

Site Z: No internet access, up to 2 big arse antennas
Internal gateway of 10.100.50.1
Netmask of 255.255.255.0
I imagine that fixed IPs are in order internally, one machine with
fixed IP of 10.100.50.9.
Two Z-sides of bridge (one to/from A, one to/from B).
No idea as to the IP of these:
A - 10.100.50.2
B - 10.100.50.3?
or
A - 10.100.48-3
B - 10.100.49.3?

The main computers at each site need to access the internet, but also
hit 10.100.50.2 over the wireless replacement at each site. Yes, it is
on a different subnet set than either site.

Is this where I learn about VLANs?

Any and all help that you can give would be appreciated. Thanks.