His typical rental period is 1-2 months, and he gets roughly double the monthly rate of a standard year long renter.  

I am totally new at this and have no idea what "standard" numbers are for this kind of a deal, which would be 10-20% down with a certain monthly amount and balloon payment within about 5 years.

Use standard purchase contract and include addendum with seller financing terms.  

HUD provides standard form for it.

Make sure you have a detailed scope of work, acknowledgement of standard codes with a willingness to bring the property into code and I think you will find them very helpful. 

Once you have an FHA loan you are not allowed to have another FHA loan unless relocating a great distance for work.Now that you have exhausted your Low Down Payment Options (Portfolio Conventional and FHA Conventional and FHA) you will need to use a Standard Conventional Loan and put 20% down to get your 3rd Duplex.The sky is the limit...you could continue to buy properties and build an empire or just stop there and enjoy more free time...I have friend who quit his job and just manages his properties now.Keep in mind that this 4 step process I outlined takes time (anywhere from 1 to 10yrs depending on the person) so keep the big picture in mind but don't let it stress you out too much.

Add $180/month for property taxes and the total is $1,097/month - an annual total just $564 higher than the 2015 standard marital deduction of $12,600!

and am talking about paying a initial down payment and then paying the rest of the amount in some time and I need to know what is that standard time- is it 3 months or 6 months or an entire year ?

Only negative I see now is that I don't know how I'd claim the mortgage tax deduction since it wouldn't be in my name.

High interest non-tax deductible credit card debt?