It shouldn't be expensive to draw up, but you will want to be able to provide solid evidence of the deal to IRS auditors, estate attorneys, siblings, or any other interested parties in the future. 

I found the bank info from our county auditor site, should I contact the bank?

I did a refi, last year to lower the rate on my school loans, so currently i owe 160k.I don't think i would be able to afford a 2-4 unit and live in one, but i would do that.I been saving and have some funds for the maintenance, vacancy and expenses that would cover most things, unless something drastically happens to my current unit.I'm an operations auditor for a large private company, the income combined income of my wife and i, would be able to pay both units if i had vacay and just had to cover both.

You've got the IRS and Auditors breathing down your back. 

From what I understand, the appraiser will need to argue the assessment when I go before the county auditors.

If you aren't using financing ask your realtor for comps.Get insurance quotes before you make an offer.Verify taxes on the county auditor's site.  

I have heard of auditors actually going to a home and inspecting and if a television was thereit had to be justified as a business activity, otherwise the room was considered an additional living space and occassionally used for business thereby disqualifying all the deductions.I have been through this several times when I used a home office long ago.

If the audit leads to a different outcome, and your point is still arguable, appeal to an investigator or higher level auditor.

Check the value the county auditor site has listed right now and if it's lower than your purchase price plan to adjust accordingly.

Depending on how much business you actually get in renting that house out on AirBNB, it may be an issue to sell that to a tax auditor should you ever need to.The second issue that I had was that there are rules about equity positions between former and exchanged properties.