Need advice re chaining http proxies

Requesting advice for creating a transparent proxy setup that contains
these proxies:
Privoxy
Apache
hapv

Desired:
Privoxy blocks ads
Apache, (proxy is on) caches pages
hapv (a new [ver 0.79]) virus scans incoming http.

1) How can all LAN users be forced to hit the first proxy?
2) What should the order of the proxies be? Why?
3) What ensures that only incoming requests will hit the proxy chain?
Apache must continue to serve outside generated requests.
4) Should one or more of the proxies use localhost rather than an
internal IP?

I want hapv on the firewall machine if possible because hapv uses a
specially mounted ("mand") hard drive partition to store parts of the
file downloaded and the firewall machine is the only one that has
unpartitioned space.

Setup:
4 computers behind a Linux firewall
************
| FIREWALL |
************
| | |---->[ GoToMyPC demo ]
| | |----->[ Winblows ]
V \--------|
************| |
| Apache | |
| Postfix | |
| FTP server| |
************| |
\-->[LAN] |
V
******************
| NNTP |
******************

The Apache+Postfix+FTP computer has forwarding on and SNATs lan users.
All LAN machines have gateway set to this machine.

Each of the above proxies has been installed and tested. Setting a
browser - to proxy to each in turn - behaves correctly.

Thanks for any examples, suggestions, Etc.!
--
buck