I have heard mixed responses on whether to charge both first and last month’s rent and a security deposit, or just first or last month’s rent and security deposit.

HOWEVER, I'm reading a lot of mixed reviews on whether I'll be able to put the property in an Ohio LLC (we won't be the primary owners of the LLC, since it will be owned by the Holding LLC.

No - "DSCR Loans" are for residential investment properties only (with only a very minor exception for small mixed use properties that are majority residential in nature in every way)

Would also recommend getting a business checking account and credit card so that your personal expenses aren't mixed in with rental property income/expenses.

This can include emails, letters, inspection reports, and any other relevant paperwork.

Make sure you have proof of funds or a pre-approval letter ready to submit with the offer. 

Purchase price: $250,000 Cash invested: $50,000 Mixed use building in Brooklyn that my family has owned for more than 25 years.

If mixed in with your deposits and reserve funds, you may accidentally spend money you shouldn't have.

All letters, a mixture of letters and post cards or what and how often?

I looked into possibly obtaining loan and they required 1% of loan amount for a pre-approval letter and no fund of the 1% fee.