My father took out the loan with my mom as a co-borrower.

My thought on a co-signer is that it just puts extra pressure on the primary signer(s) to do the right thing and pay the rent for fear of negative impact on and feedback from the co-signer if not...

I live in a co-op building and my neighbor was denied by the board when he tried to purchase his unit.  

If you need to do the paperwork yourself, you will need a co-op lease agreement.

I am a co-founder of a up coming clothing brand and we are looking for a place of an office and a storage for the clothes.  

On a serious note, why not just act as a co-signer rather than lender?

When it comes to Multi-Family properties, a CO is required and if the property is not owner occupied, Philadelphia requires a Commercial Activity License. 

So the agents I wanted to add to my term wanted to qualify me - "How's your credit, how much cash do you have to put down, do you have a co-signer?"

It is a Co Op there is no HOA however there is a recurring monthly maintenance fee of $287 per month, the co-op shares which are based on the price of selected lot or home sites current pricing is set at $20,000 per lot, the homes are manufactured or mobile homes new ones start at $100K .......the $28K she is asking for includes both the co op share/lot and home.

Property A could be acquired with a co-investor through a tenant-in-common ownership structure.