As always, there is a lot you haven't told us, such as your budget, how many people participate in your
"discussion-type web show" (how many are on-camera and need to be miced), how much audio experience you have and what audio gear you already own.
The Canon XF100 does support 16bit/48kHz audio and has XLR audio inputs, so using a separate audio recording system is not strictly necessary. A good mixer will be a definite improvement over the cameras internal pre-amps and allow the PSM to control the audio without interfering with the DP. However, a completely separate audio recording system will allow the DP and PSM more freedom as they are not joined by a cable. One more factor to consider is that the separate audio recorder will most probably have 24bit capabilities, which will give you much more audio headroom.
As far as which mics to get it is usually dependent upon the specific situation. For sit-down interviews a lav is usually preferred, one for the interviewer and one for each subject, very much like what you see on a Discovery or History Channel show. Interviews in the field are done with handheld mics as you see done in newscasts. Other field work is usually done with a mic on the end of a boom-pole.
Suggestion number one - Hire a professional, or at the least hook up with a knowledgeable audio up-and-comer. You probably won't do this, so on to
Suggestion number two - As you will be dealing with varied situations on a regular basis I would suggest that you rent rather than purchase unless you will be doing these on a very regular basis. Trew Audio has a location relatively near you (Nashville) and delivers anywhere in continental North America.
http://www.trewaudio.com/rentals/nashville-mixers/
They are very affordable and have excellent support.
You probably won't do this either, so if you must purchase give us a budget and your specific priorities and we'll see what we can come up with.