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?

Are you cash flowing the property?

I want to build more confidence in bigger projects: flooring, tile, electricity, etc.

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 .

That's Only cash flow.

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.  

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.