N.,
I am also a Realtor and have dealt with a lot of septic issues. The law regulating private septic in Mass is called Title V. A good source of information is:
http://www.mass.gov/dep/water/wastewater/septicsy.htm
A home is required to have a Title V inspection before it sells, but there is no requirement for the seller to fix it if it fails. The important thing is to have a Title V contingency on your offer so you can get your money back if it does fail. Sometimes the seller can't afford to put one in, so they sell it "as is". Mortgage companies won't give you a loan without Title V, but a cash buyer could buy the house and put it in themselves. Sometimes it's just a minor repair, but a new system can start at $20K and go up to $60K!
Check with the town you buy your home in. Most do not allow garbage disposals with private septic. A good size tank is 1500 gal. That is the requirement in many towns for a 3 or 4 bedroom home. The frequency of pumping is really dependent on both use and your system. A lot of people with a lot of laundry and showers with a high water table in the area will probably want to pump once every year or two.
I urge you to use a Realtor when you buy your home. He/she will know all of the above and guide you through the process of buying. Good luck,
D.