too many unknowns. judging by the monthly rent per side, this appears to be a C are at best. are you prepared to be a landlord, or are you going to outsource to a PM for at least 10%/month? if this was such a banging deal, why are they coming to you? the color scheme tells me this was/is a flipper's house. so what happened to them? did they go bust or did they find something that they wan to offload to a sucker like you? (no offense)
even if everything checks out, the 20k "estimate" to complete the other side was pulled out of your behind. i'd triple and quadruple check the numbers with actual contractors (if you can still find those), unless you're going to be doing a diy on it yourself (then double your timeline without cash flow on that side). your mind is overshadowed by potential success without giving any consideration to what can go south on this and quickly. the outrageous financing alone should give you a pause. in the end, you'll be out $45k-50k cash (best case scenario) to cash flow $100/mo? (if not negative, as your piti will be around 700-800/mo) forget about that "ARV" also. with the way things are looking right now, those are not real numbers either.
all this to not discourage you, but to come to the table prepared for the twists and turns that are surely going to be there waiting for you.
