Used vehicles do not qualify for the energy credit.   And if your only income is Social Security you do not have to file a tax return unless you had tax withheld from your SS or if you have a 1095A for having marketplace health insurance.   

SSI =  Supplemental Security Income, which is neither taxable nor reportable.  Also, since it (unlike regular Soc Sec or SSDI) is needs based, your purchase of that a vehicle could impact your receipt of those benefits.