UGH.Deanna yes the interest calculation can be complicated. 

Could get pretty complicated and you'd need to find partners you trust.

I have a pretty complicated story as to why I started but in short I began working for an accounting firm and I realized everyone with significant wealth had one thing in common,  they all invested in real estate !  

The foreseeable problems with any of these 3 options, as opposed to purchasing with straight cash are:  1) the seller might not want two people coming through, bothering tenants 2) he might shudder at the sound of paperwork and complications from creative financing or institutional lending and back away entirely.  

These are risky and complicated investments, not something you just order up online.  

Unless you have good construction experience, stick to something that isn't too complicated.

It's going to be too complicated with a partner.

As far as doing a wrap-around mortgage, the transaction becomes more complicated and also, it might trigger the "Due-on-Sale" clause the seller has on their mortgage.

@Lisa Malservisi Cornillot   It's pretty complicated!   

I'll mention that I just closed on a duplex yesterday that was off market and complicated but worth the time/effort to get.