Hi Craig,
Marion county property cards are not the best source of data, but they do provide some valuable info. The information in them is determined from exterior drive by's and walk arounds by Marion County tax assessors, and previous known information (like building permits and inspections).
On the summary of improvements, the grade is simply used to determine the replacement cost of the building. If an improvement has a grade of "C." then the replacement cost is 100% of the assessed value for insurance purposes in case of total loss. Grades of B, A, AA, and AAA all represent better than standard build quality, and a grade of D or E represents lower quality. Here is a link for more information on that: http://www.in.gov/dlgf/files/2011_Appendix_A_Final...
To find property taxes, I recommend looking at the tax history on zillow or realtor or trula. A quick and dirty (conservative-ish) calculation for Indy investment properties is to look at the assessed value and multiply by 2.5% per year. Evan in the post above had a more precise method.
Hope this helps.