Are you cash flowing the property?

He is willing to do a 25 year seller finance on this deal after accounting for vacancies, maintenance, capital expenditures I should be looking at around an 18% cash on cash return ROI is that a good cash on cash return for a mobile home or should that be higher?

I could pull a heloc on my existing primary home (in Florida) to purchase the land and use towards the build along with cash (50/50).

Yes, sign a listing agreement with a seller, then bring that lead to cash buyers on the MLS, and get paid a commission. 

I have a vacant property in Lake Worth Fl, I would like to build a duplex or triplex on property, Property is paid for, I am cash poor, approximately 5000.00 and credit score is fair .

I have a cash flowing MF portfolio (about 80 units) and I will be leaving my corporate job soon to 1) be a better Mom to my kids and 2) focus on growing my RE portfolio.  

That's Only cash flow.

The OLD rule only allowed you to leverage your cash and only put 3.5%-5% down payment on duplexes.

Purchase price: $275,000 Cash invested: $294,000 Seller was behind on pay HOA payments and in risk for losing the property.

it's crazy to tie up a million cash to build a duplex. you can leverage that and build so much more units. ohio is one of the cheapest markets in the country to build and I'd pass on this deal if it's a million which I'm assuming it is of cash, with that amount of money you could be the majority partner in a 50+ unit deal with non recourse debt on a HUD 221d4 loan and have more than 50% equity. super low risk, urban core, non suburban deal in Columbus Ohio.