Not exactly sure how to reply as you cover a lot of ground. The DVX is simply the base unit for the Response Point Phone (RP) system. You really need to decide how many phone lines you will need. Keep in mind that the fax requires a dedicated analog phone line (POTS) and will not be a part of the RP system and that you need to consider that lines are required for both inbound and outbound calls. You also need to consider if the voice (not fax) lines will need certain features such as caller-id, hunt group, etc. Once you have decided what your requirements are, then you can shop the providers to see if they have what you need and can combine services in order to give you the best price. If you go with the local phone or cable TV company to provide plain old telephone services (POTS), you will need an ATA such as the Dlink-3104MS as well. This device basically converts your standard analog phone line into digital packets for insertion into your network and control by the DVX. You may also be able to go SIP, but again this is simply a decision based upon your needs and $. While some people may make devices that will interface a fax machine to a digital stream, I have not yet seen one work at a reasonable cost that works as well as a dedicated analog connection. Of course, there may well be some folks out there now of which I am unaware.
THe DIR655 is an excellent router, but am not certain how you intend to use and why the need or intent with the wireless as you also mention a switch. Sorry, about being a little vague, but need more info.