D1 and D2: Federal direct unsub–yes; grad plus–no. (Although D1 and D2 have classmates who have.)
That depends on the total amount borrowed over the course of med school plus any accrued and capitalized interest;, the annual interest rate (Fed Direct interest rates float and are readjusted every July 1) and what repayment plan your D chooses to use. Students have the option of consolidating their total federal direct loans after graduation. They are allowed to consolidate loans ONCE without incurring a penalty.
There are a variety of repayment plans, including Pay As You Earn (PAYE), Income-contingent repayment (ICR), income-based repayment (IBR), graduated repayment and extended (25 year) repayment, as well as the standard 10 year repayment option.
See: https://www.aamc.org/services/first/first_factsheets/112294/loan_repayment_choices.html
There are also some repayment for service programs offered by the National Health Service Corp and by some states. The programs are for primary care fields only.
Federal Direct and Grad Plus do not require a parent to co-sign. Some med schools (privates in particular) ask about family finances in order to assess an required family contribution.