One tenant has been there for 20 yrs, and my aunt has barely raised rents in all that time....last time she did it was $10!   

Even if it was down to bare studs.

I would assume 70-100 thousand as a bare minimum to purchase something on land with utilities hooked up.

I had the same issue with my 1st purchase of a 12 unit building during first 2 years - not as good cash flow as advertised and barely making a profit after mortgage. 

Is there a vapor barrier under the floor or is it just on the bare concrete?

Barely positive cash flow with a conventional loan.

I read elsewhere that others bump up the rent for the extended lease option, but as they can barely cover the rent as it is and I actually want them to stay another month, I'm thinking of leaving it as is. 

Have multiple exits in mind starting with your first preferred method followed sequentially by the other exits prioritized if your first exit strategy isn't going to bare any fruit.

So if your barely getting by you may like to reduce your expenses, and build reserves first.

It’s easy access, with bare walls and an unfinished basement.