Rebecca
Need is such a strong word. Assuming that you are currently doing 35mm, you probably have a 50mm lens in your enlarger. Using this lens will allow you to enlarge a circular part of the middle of each of your 6x6 negatives. An 80mm lens will let you get the whole thing - and as such would be strongly advised.
I believe that the M370 has a kit available with all of the bits and bobs to do 6x6 in it. In particular what you are going to want is:
80mm lens
New condenser, or spacers for the existing condenser
New negative carrier.
You can probably find the kit at anyplace that sells the Durst Englarger, or you could find the parts on E-bay.