Can we claim a medical deduction for the conversion portion of a mobility van on 2016 taxes based on the 1-15-2016 on the bill of sale date?

We paid for the converted van in 2015 and took ownership in 2016.

Yes, you can claim the "conversion" based on when you paid for the conversion. If the conversion was paid on the bill of sale, it is a medical deduction for 2016.  You can include in medical expenses the cost of special hand controls and other special equipment installed in a car for the use of a person with a disability. You can include in medical expenses the difference between the cost of a regular car and a car specially designed to hold a wheelchair.