It was built 1915 and the bones are super solid but the last major work was done around the 70’s.

I believe Erie distributes their products thru independent agents so go back to your current agent and ask them to review your policy and shop. 

It seems like he wants conventional terms but a portfolio product.

Then start researching construction loan products in your state incl. fixed rate one-step because interest rates are rising, and find one that let's you borrow WITHOUT the land. 

You are free to purchase the product they offer, or you can not purchase it and buy a product from another vendor.

I've tried searching for such a product with no luck.

Personally I would probably not invest in them unless I owned a majority share of the complex and could control the HOA.

There are so many lenders who offer this product but interest rate will be higher.

I know the majority of the information I need to determine what I'm willing to pay (income, expenses, back taxes, etc), but I need to find out how much they bought the note for and what kind of margin they may be using as a baseline. 

I know some products that will actually let you count "expected" income from a rental property...meaning an appraiser will appraise it and also give an estimate of monthly income that the property would bring in that can actually be used towards your total income.