There is a lot of information on the homequiltingsystems group at
http://groups.yahoo.com . After you join, check out their "files" section. They have some very informative threads now and again also. A recent one was about being intimidated after spending a lot of money on a setup. Someone said the best advice they had gotten was to spend a year researching and trying out machines before actually purchasing one.
My understanding is that midarms start at 13" harps, and longarms start at 18".