It is a pretty sweet deal at first glance. But would need estimated proforma to justify.
Just wondering, how did you estimate 100k rehab cost on a 1,400 SF duplex? That's over 71$ per sf rehab, which puts it at the higher end of rehab estimates with very high quality material.
Regardless, you have to look at it as 4 units on one property. Numbers have to make sense for when you live in it, just as much as when you move out. At 349k purchase, you're looking at under 1800$ per month mortgage payment. I would factor in, how much rent you'll get for one of the duplex units + the mini house. Does it get close to 1800 per month? If so, you pretty much have your mortgage paid for and you're just paying for expenses/insurance/property tax instead of rent. Of course, this doesn't factor in the renovation cost to bring the duplex to rent ready status.
Also, are the similar houses in the neighborhood that are going for 320-390k all the same criteria (main house, duplex, with mini house or 4 units, I would doubt that). If it has been sitting on the market for several months, more than likely, you can offer a lower price than the 349k. Find a number that works for you, worst that can happen is the seller says no, or counter.