network

Clarifications

..sharing. (at least until you work out net details on VPN equipment):
https://www.dropbox.com/

Each location may think about getting a free account with no-ip.com so connections can be made economically rather than using static IP's (although you did say all are using the same ISP.. so you may want to check with them on a block of 6 static IP's..)

SMC and DLink both provide DDNS to No-ip.com so you can purchase 6 of the same VPN model for simplicity. Then you'll each set 5 static connections authenticating to each others No-ip host names with a pre-shared key and AES encryption.

..that look like they fit your application.

If I was in your place, I might set them up like this:

Configure all 6 up at your office, enable a remote management port (use a good password), enable DHCP with each set to a slightly different subnet, and use 2 NO-IP free accounts (each with 3 hosts) to DNS-authenticate the end-points, and save the config files to your HDisk.

Afterward, take 5 units to your closest biz partner and individually test connections using his ISP user/password and get him to renew his IP each time to ping test all subnets. When the tests are complete, take the remaining 4 to the rest of the biz partners and charge them each $250 for the hardware and setup.

If you get the wireless models, charge them a fee each time you have to do any remote management - keep good documentation and renew your backup files each time you make changes (that way, if someone hits a reset button, you'll just require a short phone call to set up the ISP password and enable remote management so you can load the config again).