If the home is a Pre HUD, built before June 15, 1976, you may not be able to find any long term conventional loans. Additionally, most manufactured homes built before 1979 used caustic and carcinogenic chemicals i.e. formaldehyde and asbestos in several of building components and materials i.e. particle board sub flooring, acoustical and/or popcorn ceilings, countertops, insulation etc.
If you're going for a Freddie Mac or Fanniemae better ask them what their parameters are first. Not sure about VA but I don't think they even ask. But most importantly, if you haven't already, you need to contact Sonoma County Building Dept first. They are known to be pretty fussy and particular about this.
Lot's of things to think about here.