I purchased my current multi unit with pre existing tenants, and it has been a great experience. 

A pre-existing structure will mitigate a lot of the issues Becca, above, mentions - particularly with zoning and permit which can get expensive and be a lengthy time-process (time is money).You'll need to find a boots-on-the-ground or a contractor that can handle multiple units - you may be able to find this resource through vetting a property management company or by joining social media groups for investors in that area.Always work with licenses, insured, and referred folks - from the contractor to the realtor to the PM etc.Best of luck to you!!

If they lease between November and mid March you are adding vacancy to find the right person at an optimum price so I would  make sure they end leases and set new expirations in the spring to early fall to ensure that.Back to asset class though think of that because if the preexisting tenant is dirty and destructive not only do you need to pay to fix the stuff their deposit won't cover but you also might have 2 to 4 weeks of vacancy before you can even start to lease it. 

The trouble of course, is that I have the preexisting relationship with them.

I've paid about 6k for new furnace and AC (pre existing ductwork in place).

.- Mom/Pop tend to be drive up, not climate controlled, no onsite professional management and site is based on pre-existing land and convenience to the Mom/Pop and not the customer.

The issued permit and progress inspection approvals on the garage demonstrate preexisting municipal approval without any variance, and demand inspections continue where they left off.

The issue though is being able to utilize pre-existing properties to incorporate the affordable housing aspect into this.

http://en.wikipedia.org/wiki/Putative_marriageWOW WOW WOW, this is a scary read - " putative marriage is an apparently valid marriage, entered into in good faith on the part of at least one of the partners, but that is legally invalid due to a technical impediment, such as a preexistent marriage on the part of one of the partners.""

What is your first conversation with them look like(creating preexisting relationship)?