What would you do if in my shoes? Writing counter offer on sell of house.

[Thomas Bybee]

I am in process of selling a house, I have received a contract to purchase but with some circumstances I'm trying to wrap my head around to decide if I want to accept the offer. This is my "old" primary residence (moved out of this residence 6 months ago and have been renting it since then.)  I would keep as rental but due to some regulations on the mortgage I am not allowed to do so, (refinancing would require a 25% down payment- which I don't have at this point- since it is a investment property now- as far as I understand). I've been given permission from the mortgage company to rent it for 6 months, but can not rent it for profit. So I'm looking to sell the property at this point as soon as I can since I can't rent for more than the mortgage payment which only costs me money (except for mortgage pay-down).

Details:

The offer is good and I will make money off of it, which is a plus of course.

Price isn't necessarily the issue.

The offer includes $20K  non refundable as down-payment. They are very motivated buyers.

The offer has an extended closing date for 6 months from now, since the couple making the offer didn't quite qualify for their loan. They (the couple and their relators and mortgage officer) are saying they need at max (will close sooner if possible) 6 months for credit repair. So they are asking that I rent the house to them for up to 6 months while this is going on (they just sold their primary residence and don't have a place to live now). 

My questions:

As a FSBO the 20k down payment can or should be written out to me as seller? Or as they have it written in contract... go as earnest money in escrow? I want to be in control of this non refundable money as much as possible in case they back out. Advice...

Also the property currently has no garage, but there is space for a garage and they are requesting to build a garage, at their expense, immediately upon moving in, during the time they are renting. I want to protect myself and protect them as well, so am wondering the legal aspects of this and what I need to include in the lease agreement if I decide this is ok, or if this is a good/bad idea.

I'm sure I'm missing details so feel free to ask if needed.  Looking for advice.